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Cancer pagurus and Necora puber fisheries are commercially important within the Northumberland Fishery especially during months during which yield of the more valuable Homarus gammarus is low. Crab fishing occurs throughout the year while the Lobster fishery has the greatest landings in late summer. Total value of the Edible crab fishery in 2008 was £243,492, an increase of £65,303 from 2006. This is due to an increase in both landings (+50.561 tonnes) and value (+9p Kg-1) (DEFRA 2008). In 2009, between January and July, fishermen in North Shields, Amble and Blyth landed 53 tonnes of crab with a value of £49,814. (MMO 2009)
Fishermen using static gear such as pots and creels target Crabs, Lobsters, Whelks and Prawns within the NSFC district (NSFC 2008). The crab potting season reaches its peak in spring/ early summer but continues throughout the year (unlike the lobster season which peaks in late summer, making the crab fishery more important during the months when lobster landings are low). Velvet Crabs were once considered a pest, but since landings throughout the district have increased, they are now a marketable.
The assessment of the fisheries will take place within the Northumberland Sea Fisheries Committee (NSFC) district which covers the sea area out to 6 nautical miles from the coast with the northern boundary being due east from the Northumberland county northern border and the southern border being due east from the south pier of the River Tyne. This area is controlled by the NSFC and is subject to their byelaws.
Fig 1. A map showing the extent of the NSFC district and local ports (Turner et al 2009)
Habitat data has been conducted using ground-truthing video surveys from 1993-1997 (Foster-Smith 1998) and this data can be used to examine areas in which Cancer pagurus and Necora puber populations can be expected.
Fig 2. Distribution of substrate categories (Turner et al 2009, after Foster-smith 1998). Hard substrate can be defined as bedrock, boulders and cobbles. Patchy substrate can be defined as a combination of smooth and hard features such as boulder and sand or bedrock, boulder and sand. Smooth substrate is defined as coarse, medium and silty coarse sand, as well as offshore silty gravel. Unknown substrate within the NSFC district is areas not covered by the survey.
Laws concerning potting activity within the area are as follows;
Minimum Landing size (130mm carapace width) - The Undersized Edible Crab Order 2000 (SI 2029/2000)
Protection of Berried (Egg Bearing) or Soft shelled Cancer pagurus- NSFC Byelaw 7
Prohibition on landing parts of shellfish (Cancer pagurus and Necora puber) - NSFC Byelaw 8
Prohibition on use of Edible Crab (Cancer pagurus) for Bait-NSFC Byelaw 9
Redepositing of prohibited shellfish- NSFC Byelaw 10
Permit to fish for and sell Lobsters, Crabs, Velvet Crabs, Whelks and Prawns- NSFC Byelaw 13
Pot Limitation (800 pots with a permit, 5 without) - NSFC Byelaw 15
Any person or vessel operating in the district with 5 pots or fewer requires pots to be marked with a white tag issued by the Chief Executive of the NSFC and may land 1 lobster and 5 crab per day- NSFC Byelaw 15 (3)
According to the MMO, North Shields (the administrative port) has a fishing fleet of 302 boats registered from 1st November 2010 operating from various ports within the NSFC district and some from other areas. Of these 302 boats, 235 vessels have an overall length of less than 10m, which in many documents are a loose definition of an inshore fishing fleet. 67 vessels have an overall length of over 10m and are located at the larger ports of North Shields, Blyth, Seahouses and Amble, whilst the smaller ports are exclusively boats less than 10m in length (Cullercoats, Craster, Berwick etc).
In 2008, 120 individuals applied for yellow tags for their pots or creels and these will be for full time and part time fishermen. Well over 100 individuals have applied for the white tags, showing that they are fishing 5 pot/creels or less. These are recreational fishermen and are not permitted to sell any of the catch.
Of the vessels under 10m in length, 194 have a shellfish licence which is over 82% of the under 10m fleet. This shows the economic importance of the shellfish fisheries to the local economy as it supports this amount of vessels. These smaller vessels are suited for crab trapping as they require few crew members to operate (many are manned single handed, NSFC pers. comm.), use little fuel, are manuvable and can operate in shallow waters. These elements also allow the boats to have home ports such as Cullercoats, were they can be pulled out of the water when not in use and therefore not require an area with a harbour that is constantly covered by a sufficient depth of water. It also means that the boats can be kept closer to the fishing grounds, reducing fuel costs.
Of the vessels over 10m in length, only 19 have shellfish licences and these vessels will fish for Nephrops offshore. Nephrops is the most valuable fishery within the NSFC district and, in June 2010, the MMO reports that North Shields, Blyth and Amble landed 815 tonnes worth £1,963 264. Due to the proximity of the crab habitats to the coast (see 1.2 and 1.3) larger vessels are unable to fish these shallow areas. Also the cost of such boats (fuel, maintenance and crew), the relatively low income from crab potting and the limitation on pots prevent trapping of crabs using a larger vessel economically unviable.
Cancer pagurus and Necora puber currently have no quota in place to limit the catch, due to the strength of the stocks.
1.2 Cancer pagurus Ecology
Cancer pagurus is also known as Pie, Edible and Brown crab and is found all around UK waters to a depth of 100m (Neal & Wilson 2008). The species occupy a range of substrates from muddy sand to bedrock and boulders, reaching maturity at >10 years, beyond which mating occurs.
Reproduction occurs in spring /early summer when the female has moulted (Brown & Bennett 1980) and the berried females do not eat, remain hidden and therefore are unlikely to be caught in baited pots (Howard 1982).
Taking into account the evidence from Howard 1982, it would appear that an inability to trap berried females in baited pots may introduce bias into the experiment by removing some of the female population from the capture sample.
Size (carapace width) ranges between 5-27cm and 5-19cm for males and females respectfully (Neal & Wilson 2008) with males maturing at 11.5cm and females at 11cm. According to Brown & Bennett 1980, the carapace width of Cancer pagurus individuals is related to depth, below 25m depth males and females have a mean carapace width of 14cm and between 25-55m carapace widths is 17cm for males and 15.8cm for females. These increase further below 55m to 18cm for males and 17cm for females. The minimum landing size for Edible crab is 130mm carapace width, this is measured at the widest point of the carapace and allows for reproduction (Maturity reached at 11.5cm for males and 11cm for females) to occur before the individual is possibly removed from the stock.
Juveniles remain in the intertidal zone until they are around 3 years old, at which point they move to sub tidal areas at which point they have a carapace width of 6-7cm (Regnault 1994). Between 4 to 8 years, carapace growth for males is around 1cm per year and this gradually reduces to 2mm per year between years 16 to 20. Females have a slower growth rate, around half as much as males at 0.5cm in years 4 to 8 and declining to 0.1cm between years 16 and 20 (Bennett 1979).
From this information on growth, it can be estimated that males have 2.5- 3.5 years for possible reproduction to occur and females have 3-4 years before they reach a legal size and can be removed from the fishery. However it is very difficult to accurately age crabs because there are no growth rings due to moulting of the exoskeleton so this is very inaccurate.
The diet of Cancer pagurus consists of a variety of live molluscs and crustaceans as well as carrion (Neal & Wilson 2008). Crab fishermen use whole fish that are cheap like plaice or skeletons of processed fish to bait the pots.
1.3 Necora puber Ecology
Necora puber is also known as the Devil Crab and Velvet Swimming Crab and is found on all British and Irish coasts. Found on rocky shores in the intertidal zone and shallow waters, most abundant on moderately sheltered coastal areas. Necora puber has a maximum carapace width of 8cm (Wilson 2008). Virtually all of the Velvet crabs caught in the Northumberland Fishery are exported to the European market, especially France and Spain were prices and demand is higher (NSFC 2008). The fishery of Necora puber in the UK has become much more valuable since a collapse in the Spanish Fishery (Henderson & Leslie 2006). Minimum landing size of 65mm is observed with Velvet crab.
By observing Neal & Wilson 2008 all sampling should take place in depth <100m and a range sampling locations with different substratum should be sampled, taking into consideration they need to be in the same depth range.
2.1 Trapping of Crabs
Fishermen operating in the NSFC district use pots that are designed to trap crabs and lobsters. There are a variety of different designs of pots in regard to size, mesh diameter, number of entrance holes and construction materials. There has been an increasing change from wooden pots to steel pots as they are more durable and do not require weighting within the pot. The diameter of most pots is Pots are laid out in strings consisting of multiply pots and have weights and identification dahns (weighted poles with flags that mark the position of the string)
Fig 3. A simplified diagram of a string of pots layout. The dahn is used for identification of position and ownership and this is the point where the pots are hauled onboard. The weight prevents to the pots from moving about in rough weather that can result in damage or loss of pots and also the damage/ death to the catch. In commercial fishing, a string typically contains 20 to 40 pots.
Potting vessels have a hydraulic winch that is used to pull the pots from the seabed and as pots are hauled the contents are removed, then baited again and stacked on deck. Once the whole string has been hauled, the dahn is thrown back overboard, followed by the weight then the pots. This is done as the boat is moving forward to ensure sufficient spacing between pots (usually 10 fathoms) so that a good area is fished. Strings are always laid in a North-South orientation as this helps other fishermen of the exact position of the string and reduces the risk of the pots being caught up in trawling gear. The pots are usually left to "soak" for 1-2 days in the summer, however due to poor weather conditions in the winter they are often left in for 3 days plus. Fishing efficiency reduces as soaking time increases, as the bait quickly reduces in quality due to decomposition and consumption by the occupants of the pot (NSFC pers. comm.).
Bell et al (2003) define the area that each trap has an effective area fished per trap as 7776m2 which has a large degree of uncertainty as it is derived from other species.
2.2 Study area
The sampling locations will be selected from areas of high potting activity, shown in Turner et al 2009. These areas are hotspots for potting activity, described as pot months which are expressed as;
No of pots fished per month*No months fished= pot months
Fig 4. Distribution of potting activity Oct- Dec (Turner et al 2009)
Neal & Wilson 2008 define a suitable habitat for crabs as below 100m depth with a range of substrate types. This definition was expanded more by Ungfors 2008, to between 10m and 40m depth with a substrate of bedrock, stone, sand and gravel. However this is from Swedish waters but there appears to be no evidence to suggest that the habitat preference for crabs within the NSFC district will be different.
The areas of high potting activity will be cross referenced with charts showing the depth ranges within the district. This will provide a range of sampling locations that will be cut down to about 4 sites, following the methodology of Bell et al 2003 of 2 weeks for each site. The site also needs to be within an appropriate range of North Shields, where the NSFC patrol vessel is stationed, to reduce the amount of time at sea (reduces costs and risk).
Ungfors 2008 also states the effective fishing area (if each pot is 100% effective) which is calculated as 2293Â±1137 m2 corresponding to a circle with a radius of 26.6Â± 6.3m.
2.3 Mark- Recapture method
Mark- recapture experiments have been used as a tool for measuring population size for over two centuries (Seber, 1982) and are useful in such a study as this, due to difficulty in observing the organisms because of the depth of their respective habitats. This method is preferable as it uses the same equipment and methods that fishermen employ to capture the survey species.
There have been a number of similar studies carried out on other crab species and in other areas that can be used as a framework for this experiment to follow. The principal methodology that would apply to this survey would be the method employed by Bell et al 2003.
The methodology follows the aim for a mark-recapture short term, continuous trapping experiment as the method of population estimation focussing on Cancer pagurus population on the Race Bank off the east coast of England. Firstly the Jolly-Seber method was considered.
The Jolly-Seber method (Jolly 1965), (Seber 1965) is applied as a mark-recapture method to open populations in which death, immigration, emigration and recruitment are possible. The population that will be studied is an open population due to general northward migration of Cancer pagurus females on the east coast of England and Scotland (Eaton et al 2003). This has been explained as a response to the southerly transport of larvae by hydrological cycles, that flow southwards down the North-east England coast and move offshore at Flamborough Head. This is supported by evidence from Edwards, 1967, who observed an increase in mean carapace width from south to north up the east coast. It is also observed that there is an offshore movement by females offshore to spawn around November and an inshore migration that when the eggs have hatched. This will not affect the survey that will be conducted even though it will occur in November and December as berried females do not eat and therefore are unlikely to be caught (Howard 1982). Males are generally sedentary and are unlikely to move away from the study area.
In Sweden 1967 and 2003/5, the Cancer pagurus population was sampled and monitored in a tagging experiment. In 2003, 3749 individuals were tagged and 33% were recaptured and in 2003/5, 8110 crabs were tagged in 5 locations, with 8% recaptured. These studies showed mean distances travelled for females as 6.4-21.7km and males as 1.5-8.8km respectively (Ungfors 2008). However this experiment showed a movement south and north, which may be due to larval dispersal being affected by currents that differ from the currents that are observed down the North-East coast of England.
The Jolly-Seber method has a number of assumptions
1. Every animal present (marked or unmarked) in the population at the time of the i-th
sample (i = 1, 2, â€¦, k) has the same probability of being caught (i.e., constant pi for
all animals in the population),
2. Every marked animal present in the population immediately after the i-th sample has
the same probability of survival (Ï†i) until the (i+1)th sampling time (i = 1, 2, â€¦, k-1).
3. Marks are not lost or overlooked.
4. All samples are instantaneous and each release is made immediately after the sample.
5. Immigration cannot be separated from the birth and death without additional
6. All emigration from the sampling population is permanent.
These assumptions can be applied to Cancer pagurus populations however there is no evidence to suggest that, for example, smaller individuals have the same survival rate as larger individuals in respect to predation (larger individuals may be too large to be palatable). Also that an individual that has already been captured once has the same chance as an individual that has not been captured, regarding whether a captured individual, having eaten some of the bait, is as attracted to the baited pot as an individual that may not have eaten. Also the phenomenon of an individual becoming trap-shy or trap-happy after the first capture, which would alter the probability of the individual being trapped again in comparison to an individual that has not been trapped.
The methodology that Bell et al (2003) used is designed to not alter any of the assumptions that have been made in the Jolly-Seber method of population estimation. The method of marking the individual, by using epoxy-resin paint, was non toxic to the individual and cured underwater. This enforced the assumption that the marked individuals had the same probability of survival and that the chance of the mark being removed was nil. Also the method of recording every individual meant that the chance of a marked individual being overlooked was also nil. However by using a white paint on the carapace that is upwards facing, the ability for the individual to remain camouflaged may be reduced which may lead to greater mortality compared to non marked individuals. By using the crab pots as the trapping method, the chance of an individual becoming trap-shy is also reduced as the pot has no stimulating features such as moving parts which reduce the stress to the animal. In the methodology, Bell et al use a range of baits for the pots. This could introduce bias into the sampling if a preference to particular baits (i.e. frozen compared to fresh and different species), by making pots in one area more attractive than pots in other areas, which overlooks the assumption that the probability of all individuals being trapped will be the same.
The methodology does incorporate that the sampling is dependent on safe working conditions and allows for opportunistic sampling, which is why it will be the methodology employed within this survey. Also the sampling period of an initial 6 day soak to generate and then hauling/setting pots at 2 day intervals over a 2 week period (giving each pot a 4 day soak) allows for catch rates to be optimised.
This very localised intense survey will be the basis for the survey that will be undertook but entire survey will be increased by including more sampling locations as the study hopes to use a number of sites within the NSFC district that have been classed as hot spots for potting effort (Turner et al 2009).
Turner et al also used a different methodology in the study of the Homarus gammarus fishery within the NSFC district. The results were obtained from the NSFC returns database, which contains the monthly returns from vessels including pots worked per month, number of landings and also weight of landings. Data was also collected from NSFC patrols that observe potting vessels geographic position. By using this data, results showed on a density map, the potting effort, vessel sightings, weight and number of landings for 3 month periods throughout the year. Also the landings data was analysed to produce a sample size composition and proportion of size classes recorded each month.
All of these would be useful statistics to employ within this upcoming study apart from the focus on the individuals that had the minimum landing size (MLS) and just above, which was used to examine whether an increase to the MLS would be required to protect the stock. This does not apply to the Cancer pagurus and Necora puber fisheries as they are both healthy (with Necora puber populations increasing in recent years to a marketable fishery). Also the study undertaken by Turner et al is not an assessment of population and therefore does not incorporate any method of population assessment within the methodology.
Another example of a survey assessing the population size and density of Cancer pagurus is from a mark-recapture experiment conducted in Kattegat and Skagerrak by Ungfors (2008). This study helps define an effective fishing area for each pot and suitable habitats/ depth range. However this study takes place are Swedish waters, which could result in differing habitat selection but there is no evidence to suggest otherwise.
Potential impacts of surveying crab populations
Handling and trapping of crabs can cause damage and mortality in both Cancer pagurus and Necora puber. Limb loss is often a result of autotomy, which is a process by which limb loss occurs to facilitate the escape from predators. Lost limbs appear again after the crab has moulted and is about half the size of other limbs and is functional. It is estimated that it takes 2-3 moults before the limb reaches full size and is therefore fully functional (Bennett 1973). Limb loss results in greater energy requirement and reduced ability to catch prey, compete interspecifically as well as remain free from pathogens.
Limb loss in Necora puber can result in loss of eggs in ovigerous females (Norman & Jones 1993). This would therefore affect the success rate of reproduction.
Bennett 1973 investigated the effect of Cancer pagurus in the Norfolk, Yorkshire and Devon fisheries. It was found that the loss of 2 chelae or 6 legs results in a maximum loss in carapace growth after moulting of 25%. This was found to have a greater impact on juveniles that can moult twice a year compared to larger crabs that moult less than once a year. The area with the greatest percentage limb loss was the Norfolk fishery with 13%. It was observed that 88% of the catch was returned due to the individuals being undersized, soft or berried and these may be captured several times a year. Limb loss results from the need to sort the catch quickly and therefore the likelihood of a crab, that is repeatedly rejected, being damaged is high. The Devon fishery had a much lower limb loss percentage of 5% due to the average size being larger, with a rejection rate of just 17%. Due to the relatively low numbers of individuals with severe limb loss (accounting for around 3% of the population), this article decides that the effect on the edible crab population can be ignored. It is important to take into account that this article is quite old and there have been increases to the minimum landing size (described as 114mm to 127mm in 1973), which would result in higher rejection rates and, therefore, limb loss.
If pots are left to soak and the sea state becomes rough, it can cause the pots to move about and roll. The constant violent motion of the pots against the seabed can result in mortality of the whole catch (NSFC pers. comm.)
It appears from these studies that the loss of limbs during trapping is inevitable and that the rejection percentage is linked to the percentage of individuals in a population that have undergone limb loss.
Recommendations for survey
This study should take place in spring/ early summer to allow for females that have been offshore to spawn to move back within the study areas. The study areas should take place on a range of substrates (hard, patchy and smooth), in 10-40m depth. These areas should be cross referenced with areas of known high potting activity to provide a fair evaluation of the fisheries within the NSFC district.
Parlour pots that are typical of the ones used within the fisheries by commercial fishermen should be used for sampling the populations and all pots should be the same so no pot has a greater probability of capture than any other. This will ensure that the results can be reliably related to the yield that the commercial fishery achieves. The pots should be set out in a string that is typical of the string layout used by fishermen although the number of pots should be less due to the amount of time that sorting the catch will take. Also the orientation of the strings should be the same, north to south. The bait should also be the same to increase reliability in the uniform performance of each pot in the fleet and, if possible, each pot should soak for the same time period. A boat will be required with a hydraulic winch to lift the pots from the seabed and enough deck space to store pots once they have been lifted. A crew will be needed to navigate the boat, lift the pots and set them again.
Once a string is ready to be lifted, the crabs will need to be analysed and have information taken from them. This information should be date captured, sample site, string number, pot number, species, sex, carapace width as well as comments such as berried, soft or damaged. Recaptures should be recorded also, stating the date and location (string and pot number). The contents of each pot must be kept separate for recording; this can be done by placing them into fish boxes that are number corresponding to the position of the pot on the string.
An example of the records should be something like this for the first caught, edible female, with a claw missing. First caught at Cullercoats (C), string A (A), pot 5 on the 07/12/10. Recaptured on the 15/12/10 at Seaton Sluice (S), string D (D) and pot 12
C A 5
S D 12
This method of data entry will be the role of one person and is easy to read and understand. The number that a crab is given is decided by the order in which they are caught, starting with number one (0001). The number should then be painted onto the crab, using a paint similar to the one used by Bell et al.