Counting Cornwall's Crustaceans: the first 12 months

Since June 2003, Cornwall Sea Fisheries Committee (CSFC) has been undertaking a survey of crustacean shellfish around the coast of Cornwall, supported by funding from Objective One and Cornwall County Council. The work is carried out by fishery officers aboard 7 inshore potting vessels, sampling normal fishing activity once a month throughout the year.

In the first 12 months of the survey, I collected the following fascinating facts:

• Number of trips made: 54, during which the following was consumed: 54 litres of tea, 108 litres of water, 108 apples and satsumas, dozens of homemade chocolate brownies and one sea urchin (not very tasty);
• Number of pot hauls observed: 13,352, of which 6,011 were sampled and their contents measured;
• Number of measuring gauges lost: 1 (somewhere in Mounts Bay, the crabs can measure themselves);
• Number of crustaceans measured: 23,363, each carapace (shell) was measured to the nearest mm and the animal sexed and examined for eggs, disease, damage and moulting state;
• Number of brown crabs measured: 17,738, approximately 75% of the overall total, ranging in carapace size from 50 mm to 245 mm (any larger and I'd need a bigger gauge or maybe a metre rule!);
• Number of spider crabs measured: 3616, predominantly on the north coast;
• Number of lobsters measured: 2008, ranging in carapace size from 53 mm to 135 mm;
• Number of crayfish measured: 1;
• Number of fingers remaining: 10, miraculously;
• Number of dolphins and porpoises observed: at least 100.

Analysis of the huge volumes of data and observations has just started but already it is illustrating both the similarities and the differences between these vessels and the ways in which they target crustaceans. Equally, there appears to be differences in the stocks themselves, for example, in the numbers of berried (egg bearing) lobsters caught. The presence of eggs was observed throughout the year in at least one location but in some areas, fewer than 3% of female lobsters observed were berried whilst in other areas, this rose to 6%. Whether this indicates differences in the lobster populations between these areas or subtle differences in the grounds fished and hence their suitability for berried lobsters is obviously difficult to conclude at this stage. However, what it does show is that a considerable number of berried lobsters are being returned to the grounds to release their eggs, which will hopefully benefit these inshore fisheries.

The survey work is also providing valuable samples for a PhD study at Plymouth University into bacterial diseases of crustacean eggs. In addition to egg samples, small amounts of antennae are being taken, to enable genetic finger printing of the lobster stocks around the coast. In time, this may help shed some light on whether there are distinct stocks present around our coast.

Information from the data loggers attached to several strings of pots has shown temperature ranges of 8°C to 17°C even at depths of 30 m. The graphs produced by these loggers show a distinctive diurnal (twice daily) fluctuation in temperature of up to 2°C, which presumably relates to changes caused by the tides. Recording this data will be useful as changing temperatures affect crustacean behaviour, for example, falling temperatures reduce crab and lobster catches quite markedly whilst at least one bacterial disease of brown crabs can only survive at temperatures below 12°C.

From my point of view, one of the most important aspects of this work is the involvement of the skippers and crew in the survey. Their good humour and patience is endless and the insight that they give into the whole fishing process will be essential to the overall conclusions of the work.

Sam Davis Senior Fisheries Officer, CSFC

Extract from HVMCA newsletter No.30 Spring 2005