Extraordinarily Crabby

Carcinus maenas © 2018 Duncan Greenhill
Carcinus maenas © 2018 Duncan Greenhill

I think it was the first creature on the shore I learned to identify; certainly the first I learned the Latin name for: Carcinus maenas, the green crab or shore crab. My family lived in Birmingham, which is about as far from the sea as you can get in the UK, and we went on holidays typical of the early seventies: a week in a caravan on a windy headland site, and the days spent at the beach.

I would head down the shore to explore, bucket in one hand, a nylon net with a bamboo cane handle in the other. Tides didn’t matter unless they were rising; as long as I could reach a rock pool, any rock pool, I was happy. It would be a long time before I would find a crab that was not a shore crab; zonation was still a mystery to me, and both the velvet swimming crab, red-eyed and aggressive, and the edible crab, its claws black-tipped, lived lower down the shore. The shore crab, tough and adaptable, able to tolerate the physical conditions of the shore in a way the other species could not, would prosper higher on the shore where they would not. But also where it should not, its larvae hitching a ride in the ballast water of ships to reach Australia, South Africa, and North America.

I would turn over rocks and try to catch them as they scuttled away for a new refuge. Once on one particular rocky headland, I found a cleft in the rock too deep for the grasping fingers of a child, and a crab just visible under the overhang at the bottom. On this occasion I was without my net but I did have a small ball of string in my pocket, the reasons for which are lost from my memory. I pulled a large mussel free from a clump and threw it against the rocks. I tied the smashed shell to the end of the string and lowered it close to the hiding place of the crab. I waited. The crab began to eat. I lifted the string. Slowly, slowly to the surface until I could grasp the crab and lift my prize from the pool for a more detailed examination. I returned him a few minutes later and left the mussel as compensation for disturbing him.

Carcinus, like many common animals, fools us with its ubiquity. We disregard it. We overlook it simply because it is so common, but its life has characteristics that can draw us in, if only we look closely enough. Its diet is wide; as both predator and scavenger it eats many things. One of the favourite prey items is the mussel, Mytilus edulis, but this choice has consequences for both crab and habitat.

Mytilus edulis shell
Mytilus edulis by H. Zell (CC BY-SA 3.0 from Wikimedia Commons)

Carcinus prefers a certain size of mussel, but how to breach the shell? The claws, or chelae, of shore crabs are not identical. They differ in both size and musculature. It has a larger claw, the ‘crusher’ claw, that is the more forceful of the two, but at the cost of speed. The smaller ‘cutter’ claw doesn’t develop as much force, but can close faster and is more dextrous. The claws have bumps, called teeth, on the inner edge. The claws on the male are larger than those of the female; they are needed for more than just securing a meal.

A crab finds a mussel and picks it up, taking one or two seconds to assess whether this mussel is worth the effort of attempting to gain access. If it is and the mussel is small the crab crushes it with the crusher claw and feeds. A larger mussel needs a different approach. The crab steadies the mussel in its cutter claw, the smaller end of the mussel uppermost, and applies pressure to the narrow end. The shore crab doesn’t squeeze steadily but gives a few pulses of pressure before moving the mussel slightly and trying again. The crab is an engineer – it’s not brute force that will secure the meal, but the propagation of stress fractures through the structure of the shell. If the shell is too robust or large for this approach then the crab changes strategy. This time the claw is forced between the shell halves and pieces are chipped off, gradually dismantling the shell and eating the flesh as it becomes available. In this way Carcinus can chip its way into any size of mussel. A mussel can’t find refuge from predatory crabs simply by growing larger.

The meal is not cost-free. Optimal foraging theory, the idea that animals maximise the energy they gain for the lowest cost they expend, doesn’t quite hold for Carcinus, even though as an experimental animal it was one of the examples used in support of the theory in the 1970s and 80s. Carcinus should pick mussels of an intermediate size. Too small and the mussel isn’t worth the effort of breaking in; too large and the time spent to crack it reduces the gain in energy from feeding on it. They should pick intermediate-sized mussels, but they don’t – they pick mussels slightly smaller. Remember those stress fractures? Well, they not only occur in the mussel shell, but also in the crab’s claws. The teeth on the claw become worn. The claw weakens, and in some cases can be lost completely. By feeding on smaller prey it prioritises the longevity of the claw over the immediate benefit of more food now.

Carcinus maenas underwater
Carcinus maenas – note the five distinctive ‘teeth’ along the side of the carapace and the three bumps between the eyes (CSIRO Image Library CC BY-SA 3.0)

The damage or even the loss of a claw does not have to be fatal. They can be replaced, but only when the crab moults – when it sheds its shell and forms a new one with growing room to spare. The loss of a claw may not be due to the wear and tear from feeding. Carcinus can choose to shed a claw or limb if it needs to, for example, when escaping a predator in a similar way to a lizard shedding its tail. This deliberate shedding is called autotomy.

It can take up to three moults to replace a lost claw, during which time it may have to feed on less armoured prey, particularly if it’s lost its crusher claw. Time may be critical. Carcinus has a limited number of moults and the length of time between moults gets longer as they grow larger and older. For a large or old crab the loss of a claw means that it may never fully replace it simply because it doesn’t have enough moults left. One way to speed up the replacement process is to ‘swap sides’. The old cutter claw develops into a new crusher claw and the new claw becomes a cutter claw.

The loss of a claw doesn’t just restrict the diet; for males it has other consequences. The females can only mate when they moult, and as they become ready to moult they release a pheromone into the water. Males will seek the females out and fight for access to them, and then protect her both before and after moulting. For this they need their claws. They show their dominance by holding them wide in front of them, with pincers parted, and use them when fighting. A large crab with a missing claw may just hold its own against smaller males, but against an intact male of a similar size it will lose.

After mating the female creates a cavity in the sand in which to lay her eggs and attach them to her pleopods, the appendages under her abdomen. A female can lay up to 165,000 eggs and the egg mass is carried beneath the abdomen, which she fans to oxygenate the eggs. At first coloured orange the eggs turn a brown and then a dull grey as they mature. Once hatched, the live in the plankton for two to three years, feeding on other planktonic organisms, moulting through four stages as a spiny zoea before finally moulting into a megalopa, the stage that will eventually leave the plankton and once again live on the bottom.

Crabs can’t increase their size continuously; they moult to grow. Their bodies are encased in a hard shell, and like a child with too-tight shoes, eventually they need to trade up to a larger size. This brings problems because the shell is both armour and skeleton. Without it the crab can neither protect itself nor easily move. Before it sheds its shell the crab will start to break down its existing shell, which will soften. If you’ve ever found a crab on the shore whose shell is soft, called a ‘peeler’ crab by fishermen, then that’s the reason why – it’s close to moulting. It’s vulnerable in this state: put it back where it has good cover and can hide. The cells of the epidermis will pull away and separate from the inside of the shell and start to secrete the layers of a new carapace. The crab takes up water, the pressure splitting the old exoskeleton along the sides and at the rear, and the crab wiggles itself free from its former shell. This shedding of the shell is called ecdysis, and leaves the animal with a new soft, paper-thin exoskeleton. The crab’s tissues take in more water while the new shell hardens, after which the crab expels the water and shrinks, leaving room in which to grow before it again needs to moult.

One of the common names for this crab is the green crab, but it isn’t always green. Some are various shades of a reddish-orange. All newly moulted shore crabs are green but the pigment in the carapace that gives it the green colour can be degraded by light. The longer a crab spends between moults the more likely it is that its colour will change towards the ‘red’ form, and the more likely it is to have other creatures living on its shell. The colour isn’t the only difference. The red forms have heavier and thicker shells and can tackle larger mussels than a green crab of the same size. But there is also a downside. Unlike the green-coloured crabs these can’t tolerate environments with low oxygen or large changes in salinity. That makes a summer rock pool an unfriendly place for a ‘red’ shore crab, and so they tend to be found lower down the shore or below the low water mark.

It’s been close to half a century since I first encountered a shore crab, but its familiarity hasn’t lessened my fondness for it. I still head down the shore bucket in hand but this time not alone. Two years ago my daughter sent me a father’s day card using one of those online sites that make the card from your own image. She chose a photo from our summer holiday. On the cover was my own water-wrinkled hand holding a shore crab.

Rock Pool Crab
(CC BY-SA 3.0)

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Cat around the Dragon – How it ended

Last August I set off to sail around Wales in a small catamaran for charity, and called the expedition ‘cat around the dragon’. The expedition lasted four days. Team 109 from Portishead RNLI are planning to cycle from Portishead to the RNLI headquarters in Poole. How are these two connected? Well, funny you should ask that.

Edit: In the rush to get this post out I failed to include a link to team 109’s fundraising page. The link for the RNLI donation page is https://rnli.org/support-us/give-money/donate.  If you don’t feel you can donate then please share the link.

catamaran moored in marina.
Moored at the events pontoon, Portishead marina. (CC-BY-SA 4.0)

The departure point was Thornbury sailing club, who had kindly let me use their facilities for the launch. I’d gone down the previous day and assembled the boat so that on the Saturday morning I could be dropped off with the kit, load the boat and launch a couple of hours before the high spring tide that was due around noon. The plan was that the rising tide would work against me initially, but flush me out later in the passage as I rounded Lavernock point and headed for the destination of Barry Harbour. We arrived at the sailing club late and loading the boat to a state that I felt happy with also took longer. I launched two hours after high tide, literally shin-deep in mud shoving the boat off the end of the slipway. Ten minutes later, and I wouldn’t have been able to launch at all. Reaching Barry was now a challenge, and heading into the prevailing south-westerly wind meant tacking all the way.

Progress, at least at first, was good. The boat handled well, and the GPS reassured me with speeds of between 7 and 9 knots. The compass told me the direction of my tacks were close to due west and due south. That was to cause me problems later. I passed under the old Severn bridge, and I now had the English coast on one side of the estuary only. Wales was to starboard. The voyage had officially begun. Eddies in the current slewed the boat from side to side as I approached the second Severn bridge, but strong corrections on the tiller kept me on course. The tide has fallen and there were sandbanks to both sides as I emerged from under the centre span.

The Welsh grounds is a huge sandbank that runs from the second Severn crossing for a number of miles south-west. I was aiming for a buoy called the Welsh hook that marked the edge of the sandbank where it turned west, but the GPS showed me there were still a few miles to go. Once I reached it I could turn and head towards the Welsh side of the estuary. The wind had decreased and the falling tide had slowed in anticipation of the turn in an hour or so. I’d marked the buoy as a waypoint in the GPS and monitored the distance left to go. About three-quarters of the way to Welsh Hook the distance was barely changing. I would make reasonable headway on the southerly tack, but on the westerly tack the tide pushed me back northwards and I lost most of the distance I’d just covered.

The alternative passage plan was simply to make landfall where possible overnight. The catamaran would float in half a metre of water, could easily take the ground (or anchor off), and despite it’s small size, was self-sufficient for three or four days. I would not be able to make the Welsh coast so I tacked, eased off the wind slightly, and sailed south-east looking for a likely spot for the night. I could see Clevedon pier in the distance and the curve of a shore behind. Choice made.

The wind was still dropping and the tide was against me. Sunset was less than an hour and a half away. The pier was getting closer. And then it wasn’t. I checked the GPS to confirm, and unclipped the paddle. The pier was less than a hundred metres away. I started paddling. The pier was getting closer. And then it wasn’t. So long Clevedon. I stowed the paddle and turned north, letting the tide help rather than hinder me as I looked for somewhere to land. The wind had now died completely so I started paddling again, this time with the tide. It was getting dark.

Then a call came on the radio from Portishead coastguard for “unknown white catamaran” and we had a brief chat about my intentions, before asking if I would like the lifeboat to assist me. I was already a member of the RNLI, and I’d joked before the trip that I’d be mortified if I had to call them out, the aim of the trip to be on the right side of the dividing line between adventure and stupidity. Another brief chat with the coastguard and I agreed that Portishead lifeboat would be called out.

I furled the sails and rigged a bridle for the inevitable tow, and as the light was fading fast I retrieved my dive torch and strobe from the locker (the legal requirement for lighting a sailboat of this size is to shine a torch on the sails). I could see the lights of large commercial ships leaving Avonmouth and heading seawards and clipped the strobe onto the forestay. I used the paddle to keep reasonably close to the shore but not too close.

The lifeboat arrived and checked I was OK. We had a discussion about where to take me since they (understandably) assumed I was day-sailing and had a fixed destination to return to. I explained about my trip and we decided Portishead marina was the best choice. A little while later I was moored to the events pontoon.

The forecast for the second day was a pitiful force two. Sunday would be a shore day. The cat is only 14 feet long and was moored not far from one of the footbridges across the marina so there were a few people who leaned over the railings to talk to me. I was having a lovely chat to a woman called Caroline and her daughter about the trip and mentioned that I’d been towed in the day before. “I know,” she said, “I was in the crew.”

Day three and the forecast was “westerly/south westerly three or four, decreasing two; north-westerly four or five later”. High tide was just after noon. A lock separates the marina from the channel and, slot booked, I paddled in, trailing the larger vessels that dwarfed me. A woman caught my lines and I moored to the pontoon as the lock levels became equal with the channel outside. I clipped my lifeline onto the boat: fall overboard sailing solo and even in a drysuit you have a very real, and potentially lethal, problem. I set two waypoints in the GPS: Welsh Hook, and South Cardiff. Once I reached the second I could follow the coast, turning the corner at Lavernock Point and finally head along the south Wales coast. The destination was to come ashore at Sully Island, or failing that find a suitable landing point before Lavernock point.
This was a good days sailing. Sun shining, I pass Welsh Hook. The hulls cut through the swell, rocking as the peak first passed under the port hull and then starboard. I pass South Cardiff and turn to follow the coast. I spot a pier and just beyond, two boat ramps. I pass Lavernock point and turn west. The wind is coming strongly from the west and the sea state is a lot lumpier now that the land is not shielding me. I’d assessed Sully Island as a possible stop before the trip and knew that there were many boulders there. Sully Island is only an island at low tide and unsure of how much shelter from the weather I would get I turned back towards the pier and boat ramps.

My timing was unfortunate, arriving  between the pier and the ramps fifteen minutes before low tide. My normal launching trolley hadn’t been suitable for the trip and I’d not been happy with the portable replacement I’d made so I’d left it behind. So now I would have to walk the boat up the beach as the tide rose, but at least those extra fifteen minutes were just enough to make a brew and pack the stove away before the boat refloated.

The two boat ramps belonged to Penarth RNLI, who were conducting a training session based on the reasoning that they might as well be at the station because it was Bank Holiday Monday, and a callout was very likely. I wandered over and explaining what I was doing and asked if I could beach the boat between their two ramps for the night. A little while later one of the volunteers came down the beach with a mug of tea and a Mars bar, which was very welcome. The lifeboat went out to a yacht that was aground on a sandbank near Flatholm to standby should it was needed as the rising tide lifted the yacht off. Back the lifeboat came and was recovered to the station. A little while later the crew come down the beach again. “Seeing as we’re already here” they said, and carried the boat up to the high water mark. This gave me a couple of hours to eat and sort out equipment before bedding down for the night between the hulls as the tide dropped.

Day four. The destination was Porthcawl, 22 nautical miles away. The wind was westerly, which again meant tacking all the way. Changing the batteries on the GPS and a spring from the battery compartment pinged off into the sand. I couldn’t find it. There was a GPS built into the radio but I’d have to dig into the manual to set it up. I checked over the boat and saw bare wood where there shouldn’t be any. There was a block on the upper inner surface of the hull that the cross beam fitted into. It had sheared off and the hull had moved inwards.

The damaged beam support
The damaged beam support. The glued joint had failed between the deck and beam support.(CC-BY-SA 4.0)

The hulls are pulled inwards by the tension on the rigging. The only thing stopping it would be the 4mm rope lashing on the outer end of the beam. The entire tension of the rig through three turns of 4mm rope. Losing the rig overboard would be a very real possibility in rough water and I’d been warned of the overfalls around Nash Point. I sat on the hull with my head in my hands. The voyage was over: the boat was no longer seaworthy.

Ashore on Penarth Beach
Ashore on Penarth Beach. (CC-BY-SA 4.0)

Would I have succeeded without the beam support shearing? Unlikely. I camped on the Llyn peninsula around the time I would have been passing there with the boat. The wind was blowing the tent virtually flat and the breaking waves left a wide, wild foamy border to the lee shore. It was not the only storm that week, and the weather between storms could also be described as ‘challenging’.

Will I try again? Yes, but perhaps in two or three segments rather than try to do it in a single trip, and after ironing out any other issues with the boat with weekend trips this year.

Safety equipment carried:
Drysuit, lifejacket, lifeline; permanently clipped on when under way.
Standard Horizon HX870E handheld VHF with DSC and built-in GPS.
Handheld GPS.
Imray 2600 and 2700 series charts; dividers and portland plotter.
Steering compass and handheld compass.
Crewsaver coastal flare pack.

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Cat around the Dragon

It’s been a while since my last post, so I’m opening with an announcement. Later this summer I’m going to be sailing around the entire coast of Wales in a small catamaran to raise money for two charities – the Marine Conservation Society and the Shark Trust. I’ve been wanting to do something like this for a while, and when I bought the boat off ebay last year I was pleased but there was also the thought “it’s a pity it’s not a bit bigger because then I could …”, which then turned into “hang on, what could I do?” The more I looked into it, the more feasible the trip looked. The boat is a Wharram Hitia 14. I’ve been a big fan of Wharrams for many years and I’m really looking forward to the trip.

Duncan Greenhill
Photo by Lucy Withell

The plan is to sail from the Severn estuary around to the Dee estuary over a period of three weeks from the 26th August. I’ll either be coming into marinas and harbours overnight or beaching the catamaran and coming ashore. As well as raising money for charity I’ll be taking part in some citizen science projects. I’ll be doing surveys as part of the Capturing Our Coast project, and searching the strandline to contribute to the Great Eggcase Hunt. My plan is also to conduct some two minute beach cleans and record some videos to highlight not only some of the issues affecting our marine environment, but also some of the wonderful marine life we have here in the UK.

The journey is approximately 500 nautical miles, and I’m setting a fundraising goal of £5000, which works out at £5 per charity per mile. The donations page is now available at uk.virginmoneygiving.com/CatAroundTheDragon . Please share the link if you don’t feel you can contribute a donation.

I’ll be posting updates here and on my twitter account (@ourCommonOcean) using the hashtag #CatDragon.
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What future for wild sea bass?

There are many things that can cause problems for a species. Sometimes it’s some aspect of the biology that makes them vulnerable. Sometimes it’s human pressures. And sometimes it all comes together to really cause problems for an animal. Sea bass are a popular fish for commercial fishermen and anglers, and are a popular fish for eating. Despite the fact that most of the sea bass consumed in the EU is farmed wild sea bass populations are in trouble. While assessments of the stock are not easy, ICES, the intergovernmental organisation that gives advice on sustainable fishing to the EU has said that the losses from the population due to fishing have consistently been above sustainable levels1 and also recruitment of young fish has been very poor since 20081.

Swimming Sea Bass
Sea Bass by Bjoertvedt is licensed under creative commons (CC-BY-SA 4.0)

Sea bass grow slowly and mature relatively late. They mature between four and seven years old when the males are around 35 cm long and the females are around 42 cm2. Before the 1st September 2015 the minimum size for landing was 36cm nationally2, although some regions such as Cornwall had a slightly larger size3. Not surprisingly, having a minimum size where the female fish could be landed before they had had a chance to breed is not the best route to a sustainable fishery. Sea bass behaviour also increases their vulnerability to over-exploitation. The juveniles congregate in groups in estuaries, and as adults migrate offshore to spawn (where they are targeted by trawlers4) and then return to the same coastal sites year after year5. This site fidelity means that once local populations are overfished recovery is slow, especially if there are cold winters that can kill juveniles, reducing the number of new individuals recruited to the population5. All these factors mean that the breeding population has dropped from around 16,000 tonnes in 2009 to less than 7,000 tonnes in 2015. ICES state that the ability of the population to reproduce successfully is seriously compromised below 5,000 tonnes.

So what has been the response? The minimum landing size has been increased to 42cm6 and is now called the minimum conservation reference size, which at least gives the females a chance to breed but comes at a cost for the fishermen. The fish that are above 35cm now and could have been caught under the old rules are now not available to be caught until they grow large enough to exceed the new minimum. There is also to be a closed season for six months of the year between January and June to allow the fish to spawn. For the other six months of the year recreational anglers will be allowed to land one bass per day, while commercial fishers are restricted to catch limits of one tonne per month. The EU proposes that catch limits should fall from 2,656 tonnes in 2015 to 1,449 tonnes in 2016, a reduction of around 45%7. The problem is, though, that ICES has said that the limit should be set at 541 tonnes for 20168, which would be a reduction of 80% on the 2015 figures. In other words, the limit set by the EU is nearly three times larger than what the scientific assessment says it should be. This may be an attempt to spread the impact on fishermen over a longer period, but with the stock at such a low level this may backfire. A collapsed fishery supports no one.

What else could be done? Save Our Sea Bass suggest that the lower landings limit (the ICES figure) should be adopted as a matter of urgency, and that both anglers and commercial fishermen should only be allowed to catch sea bass by rod and line or hand lines. They also suggest that the share in the 541 tonnes should be distributed in line with article 17 of the common fisheries policy, which means based on environmental criteria and economic benefits to coastal communities. The idea behind using lines to catch sea bass is that line-caught sea bass fetches higher prices. The marine conservation society has a good fish guide, and currently (November 2015) lists wild-caught sea bass as a fish to avoid. Farmed sea bass are a more acceptable alternative to wild-caught fish.

You can keep up to date with sea bass issues either from the Marine Conservation Society or Save Our Sea Bass site.


1,8 ICES Advice on fishing opportunities, catch, and effort Celtic Seas and Greater North Sea Ecoregions 2015
2 European sea bass (Dicentrarchus labrax) exploited around Welsh waters – preliminary results: December 2013.
3Cornwall Inshore Fishery and Conservation Association new minimum size for bass
4Protecting Sea Bass
5Parliamentary briefing on sea bass.
6Bass fishing: catch limits, minimum size, and where you can fish (Gov.uk)
7Commission proposes fishing opportunities in the Atlantic and North Sea for 2016

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Losing the plural

There’s been a slight change of name from ‘Our Common Oceans’ to ‘Our Common Ocean’. So why the change? Well, I went to the EMSEA 15 conference early in October and one of the first people I spoke to was Peter Tuddenham of the College of Exploration, who quickly spotted the ‘mistake’ on my name badge and argued that it should be ‘ocean’ rather than ‘oceans’. It makes sense. After all, looking at the thermohaline circulation shows the connections clearly.

Map of the thermohaline circulation of the oceans
Thermohaline Circulation

Plus, it’s the first of the ocean literacy principles: The Earth has one ocean, with many features. So goodbye oceans, hello ocean.

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No limits, no future

Recently, I volunteered with the Shark’s Trust at the Dive Show in Birmingham helping to raise awareness about their ‘no limits no future‘ campaign to stop uncontrolled shark fishing. Shark and ray populations worldwide are under increasing pressure, and the total number of sharks caught annually may be over 100 million. One study1 published earlier this year (2014) estimated that up to a quarter of shark and ray species are threatened if assessed using the IUCN red list criteria. If we think about risks to shark populations we might think about shark finning in the Pacific, or sharks being caught as part of longline fisheries for tuna2 but the problem is actually a lot closer to home.

Originally, sharks were an unwanted part of the catch (bycatch) as fishermen pursued species such as cod and tuna, but now there is an increasing trend for the targeting and retention of these bycatch sharks. They are caught for their meat, their fins and their liver oil. Unfortunately, the life-history and behaviour of sharks makes them vulnerable to overfishing and populations are declining. Sharks reach maturity late and have relatively few young compared to other fish. In addition, they often congregate in groups of individuals that are either all a single sex, or are very close in age. Should a fisherman catch these sharks it has a bigger effect than catching the same number of sharks at random from the population.

In 2012 there were 280,000 tonnes of reported shark landings worldwide, with EU vessels landing 40% of this world total, the majority of which came from the Atlantic ocean and Mediterranean sea. One issue in that this is reported landings – the true level of landings is thought to be three to four times higher3. Worse still, most of this European catch from the Atlantic and Mediterranean is concentrated on just five species4.

These five species are:

97% of all sharks caught and landed from the Atlantic and Mediterranean in 2012 are no limits species, which has been estimated to amount to 6,400,000 individuals. This is why although some of the species above are currently listed as of ‘least concern’ it’s important to make the fisheries sustainable before the populations crash due to overfishing. Once overfished, there is no guarantee that populations will recover as demonstrated by the collapse of the cod fishery on the grand banks off the eastern coast of Canada. In addition, sharks are important components of ecosystems as top-level predators. The shark trust is campaigning for science-based catch limits. Please sign the petition, and see the ‘get involved‘ page for other ways to contribute.


3 4http://www.nolimitsnofuture.org/wp-content/uploads/2014/07/no_limits.pdf

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Seawater – not as simple as it looks

It’s a typical question you’d get from a child: ‘How salty is the sea?’, and as it’s National Marine Week here in the UK it seems a good one to answer. The simple answer is that there’s around 35 g of salt in every litre of sea water, but that’s only a starting point. Oceanographers are often interested in small differences in salinity, so how else can we express this measurement? Well, a litre of water weighs a kilogram so 35 g amounts to 3.5%, but percentages are too big a unit to be useful, so we use a unit called parts per thousand (‰ or ppt). If we think of percentages as parts per hundred then it’s a straightforward conversion because 3.5% becomes 35‰ – it’s the same relationship as converting a distance in centimetres into one in millimetres. Salinity is now measured using the electrical conductivity of the water and either given as just a number (35) or with a unit called ‘practical salinity units’ (35 psu).

“Coast” by Chris Luczkow is licensed under Creative Commons (CC BY 2.0)
“Coast” by Chris Luczkow is licensed under Creative Commons (CC BY 2.0)

Another question the child might ask is ‘where does the salt come from?’. An obvious source is the rivers, but that’s not quite the whole story. If we look at the composition of seawater we find an interesting characteristic. While the salinity may vary between different locations, the dissolved chemicals that make up that salinity are found in the same proportions. It’s called the constancy of composition. The first seven major components of seawater are, in order, chloride (Cl), sodium (Na+), sulphate (SO42-), magnesium (Mg2+), calcium (Ca2+), potassium (K+) and bicarbonate (HCO3). If we look at typical components of river water we find much less sodium and chloride, and more calcium and bicarbonate, as well as additional dissolved substances such as silicate (SiO2). The difference between river water and sea water is even greater because much of the chloride in river water has come from the oceans via rainfall. So if the river water is the source of the salts in the ocean, why are the proportions so different?

The answer is to do with something called ‘residence time’, which is a measure of how long the element remains in the ocean before being removed. While sodium and chloride flow into the oceans in smaller amounts than other elements of river water, they stay in the ocean for longer. The residence times are also long compared to the time it takes the water to circulate through the oceans, which means that the oceans are well mixed, and this is one of the reasons we have the constancy of composition.

The proportions of the major components are constant but the total salinity can vary, and these small variations in temperature and salinity identify water masses that can be followed by oceanographers. For example, more water flows into the Mediterranean Sea than flows out. This is because a lot of water is lost through evaporation making the remaining water more salty and denser so it sinks. The straits of Gibraltar are relatively shallow compared to the Mediterranean and the Atlantic, so the salty water (called the Mediterranean water) flows out over the straits while lower salinity Atlantic water flows in at the surface.

There’s another important circulation driven by salt. As the Gulf stream crosses the Atlantic it heads north and cools. The remnants of the Gulf stream pass north of the United Kingdom as the Norwegian current. As sea ice forms the remaining water becomes very salty and sinks, forming a water mass called the North Atlantic Deep Water that flows south along the bottom of the ocean all the way to the Antarctic and drives a global pattern of ocean circulation called the thermohaline circulation.

Seawater has a more interesting story to tell than simply the answer to ‘How salty is the sea?’

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Upper Fowey MCZ – what’s there and local opinion

While I was down in Cornwall for the seasearch fish ID course, I went to a public meeting at the Royal Fowey Yacht Club that had been arranged to discuss management of the Upper Fowey and Pont Pill MCZ.

Sailing Boats in Fowey Harbour
Sailing Boats in Fowey Harbour.
CC BY-SA Duncan Greenhill

First of all, a little background. Upper Fowey and Pont Mill MCZ is an unusual marine conservation zone. It’s the second smallest, at around two square kilometres, and despite its small size it’s split into two separate areas. The main part of the MCZ is the upper Fowey estuary and the second area is Pont Pill, which is a smaller estuary that joins the main estuary from the east a short distance inside the entrance to the open sea.

MCZs are designated based on the features (habitats or species) within them. For this particular MCZ, there are six features listed in the designation, and all are habitats. The European eel (Anguilla anguilla) was included in the draft conservation objectives and there was a single record of a long-snouted seahorse (Hippocampus guttaluatus) recorded in the area covered by the MCZ, but the record dated back to the 1960s. Neither of these species were used as a basis for designation. There are areas of seagrass in the estuary but not within the boundaries of the Upper Fowey MCZ, which followed those of the voluntary marine conservation area (vMCA) that was there previously. It may be that there are species and habitats of conservation interest within the estuary, but not within the MCZ and so not currently protected. The six habitats are:

  • Coastal salt marshes and saline reedbeds, which are important habitats for birds and fish, producing a biodiversity ‘hotspot’, as well as providing natural coastal protection. This type of habitat is relatively rare in the south west.
  • Intertidal coarse sediment consists of pebbles, gravels and coarse sand, and is only found at a few scattered sites in the UK. The unstable nature of the sediment means that few animals can live here successfully, with sandhoppers being one of the exceptions.
  • Intertidal mud is what we normally think of when we think of estuaries – the typical mudflat that supports large populations of worms and bivalves.
  • Low energy intertidal rock are areas that are sheltered from wave action and subject to weak tidal currents, which means that seaweeds can flourish, providing shelter and protection and acting as nursery grounds for juvenile fish.
  • The fifth type of habitat is estuarine rocky habitat. Stable rock is rare within estuaries (because muds tend to dominate) and the rocky shore communities can differ quite substantially from those of normal coastlines because of the brackish water and sediment inflow from the rivers.
  • The final type of habitat is sheltered muddy gravels. These are found in areas that are not exposed to strong tidal streams or strong wave action, and the communities of animals found within them depends on the salinity. Fully marine examples of these habitats are scarce in the UK, but are found in both the areas that make up this MCZ. This habitat is important for diversity and is rich in species such as tubeworms, burrowing anemones and bivalves.

The last two habitats are the most important, and are listed as features of conservation importance (FOCI) for this site, which means that they are “rare, threatened or declining“.

Rob Seebold, who’s a marine adviser with Natural England and Sam Davies from Cornwall IFCA ran the meeting. Rob started with a presentation about MCZs highlighting that the aim for MCZs was to make the marine environment more resilient to change. Those involved in conservation often talk about ‘ecosystem goods and services’, for example, coastal areas provide us with ‘goods’ (fish and shellfish), but also services (intertidal mud protects against erosion by dispersing the energy of waves and currents). It’s the protection and sustainable use of these goods and services that enhances the resilience of the particular marine ecosystem.

There was some concern expressed by some in the audience that they would be prevented from pursuing activities they had always done because they area now had a level of legal protection that it had not had before, and whether people coming in from outside the area would ‘play by the rules’. While Rob couldn’t rule out any changes in future he did point out that the features in the MCZ were generally in good condition. The MCZ is regulated by a number of organisations, including IFCA, the Marine Management Organisation, Cornwall Council, the Environment Agency and the Fowey harbour commissioners. The next steps are that the regulators will look at whether further management is necessary and involve local stakeholders if that’s the case, but with the aim of managing features to a ‘favourable condition’ rather than extending the scope of protection. The regulators are also required to report on the status of the sites to DEFRA every six years.

Some of the concern at the meeting related to fishing issues, rather than the conservation zone itself, and Sam Davies from Cornwall IFCA responded to these as part of her presentation. An interesting point related to the bass fishery where the minimum size for landing in the Cornish area is 37.5cm (36cm in the EU), but as a member of the audience pointed out this is below the size at which they reproduce, and that locals were actually pushing for the limit to be raised to 45cm. IFCAs can set minimum sizes within their own areas so long as they are not below the statutory minimum.

It was my first time at a public meeting like this, and I was impressed. The concerns expressed were reasonable and entirely understandable in the local context, and I didn’t hear a single negative comment about marine conservation zones. And that’s important because protection doesn’t succeed through legislation, but because people protect what they value and connect with.

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It’s a fish – day two

Day two of the seasearch course started relatively early (for a Sunday). We met at Towan headland in Newquay by the old lifeboat station, and the car park started to fill with divers in various states of getting ready. Most of the course participants were diving, but three of us (including me) were snorkelling. We accessed the water down a natural rock ramp, which was much less steep than the old lifeboat slipway, and entered the water at 9.30, around two hours after local high water. As I was only wearing a summer 3mm wetsuit the cold shock was a little bracing and left me hyperventilating for a good twenty seconds, as well as giving me the start of a wonderful ‘ice-cream’ headache. Visibility was around 5m as we began to snorkel. We remained on the surface so as not to interfere with the divers surveying below us, which restricted us to observing what was in the water column or on the shallower rocks, and consequently saw mostly sand eels and spider crabs. We were joined by a female grey seal who kept us company for a while before disappearing to visit the divers. As we swam back to the exit point I could see the silhouette of the seal below me, just at the limit of visibility.

Grey Seal
Grey Seal (Halichoerus grypus). CC BY-SA Duncan Greenhill

After coffee and biscuits, we moved across to the other side of Fistral beach as the tide continued to fall to meet Frances and the other participants for the rockpooling session. This was more productive for me personally, catching a large Shanny (Lipophrys pholis, and thanks to Fiona for spotting it), and later a long-spined Sea Scorpion (Taurulus bubalis). The Sea Scorpion was a complete surprise as I ran my hands through the unlikeliest looking crevice in the rock behind where we’d left our bags and found quite a sizeable fish at about 15cm long.

Shanny (Lipophyrys pholis)
Shanny (Lipophyrys pholis). CC BY-SA Duncan Greenhill
Long Spined Sea Scorpion
Long-spined Sea Scorpion (Taurulus bubalis). CC BY-SA Duncan Greenhill

As the tide started to come in we used a seine net to sample over the sand in the surf. It was hard work, and involved coordination so that the top and bottom ropes were hauled in at similar rates, and that the bottom rope was kept low to avoid all the specimens escaping underneath. We found a prawns and shrimps, a juvenile flatfish, and a number of Lesser Weaverfish (Echiichtyhys vipera), which questioned the wisdom of so many swimmers going into the water barefoot. The day ended with pasties on the beach.

Overall, it was a great weekend. I learned a lot, in good company, and hope to return next year to do the seasearch observer course.

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It’s a Fish – Day One

Seasearch is a project involving volunteer sports divers to record the habitats and marine life around the coast of the UK. I recently attended one of their training courses (Fish ID) in Newquay Cornwall. The course was organised by the Seasearch Organiser for Cornwall, Cat Wilding, who’s also the marine survey officer for Cornwall Wildlife Trust, and the tutor was Dr Frances Dipper.

The course was being held at Newquay College and after the usual introductions, the day started with a short presentation where Frances talked about some of the main groups and families and the general characteristics of fish that would be used to identify them. In the second presentation, we moved on the fundamentals – the FLEMMS system. The FLEMMS system is designed so that you can gather a lot of information for identification in what may be a relatively short glimpse as the fish disappears into a clump of weed. FLEMMS stands for:

  • Fins, specifically the unpaired fins. How many dorsal? How many ventral? Is the tail concave, convex or straight?
  • Lateral line. Is it visible in that particular species? If so, is it straight or curved?
  • Eyes. Where are they positioned? Are they large or small? Are they bulging?
  • Mouth. Where is it positioned? Is one of the lips prominent or are the lips equal? Are there any barbels?
  • Markings. Are there any distinctive patterns, colours or spots?
  • Size. Relate the size to something more general: is it finger, hand or arm size?

We then got to practise and started with the easy option – identifying fish from photographs, although as we progressed Frances would mimic the fish disappearing by changing the slides faster. We could either make quick notes about features or make a quick diagram. I chose the diagram method. It starts with a cross to represent the fish onto which we mark the fins, lateral line, eyes, mouth details, etc. The symbols aren’t standardised since it’s an aide to our memory for identification after returning to shore, rather than a reference for others, so we might use lines or shapes for fins.

FLEMMS diagram of a haddock (Melanogrammus aeglefinus)
FLEMMS diagram of a haddock (Melanogrammus aeglefinus)

This fish has three dorsal (the first prominent) and two ventral fins which, in UK waters, shows that it is a member of the cod family. There is a curved lateral line, the upper jaw extends below the lower jaw, which has a small barbel. There is a black marking just below the lateral line. The combination of the first dorsal fin and the black ‘thumbprint’ shows that this is a Haddock (Melanogrammus aeglefinus).

FLEMMS diagram of a Shanny (Lipophyrys pholis)
FLEMMS diagram of a Shanny (Lipophyrys pholis)

This is a fish with a long single dorsal fin and a single ventral fin. The tail is convex. The head is complex, with prominent bulging eyes, and thick lips with the upper lips horizontal and slightly protruding over the lower lips. The fish is around hand size with blotchy markings. The single dorsal fin and the lack of head tentacles identifies this as a Shanny (Lipophyrys pholis).

After lunch, we went to the Blue Reef Aquarium to practise on more mobile and less cooperative fish, which included blennies, gobies, wrasse, and skates and rays.

Tompot Blenny (Parablennius gattorugine)
Tompot Blenny (Parablennius gattorugine)
CC BY-SA Duncan Greenhill

We returned to the lab at Cornwall College and had another brief presentation on some of the difficulties and confusions we might face trying to identify fish such as fish keeping fins folded down (which causes us to miscount), and changes in colouration and pattern as the fish matures or changes sex. There was a perfect end to the day with a course meal looking out over the clifftop across Great Western Beach as the surf rolled in and we wondered about conditions for the following morning.

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