Fisheries: A Cancer (pagurus) on crabs?

Adult crabs support an important fishery in Europe, where they are captured using baited traps. At the time of this paper, relatively little information was available on the pathogen profile of this commercially important crustacean species with catches exceeding 42,000 t and sale values of £60 million in European waters alone (50 % of that landing in the UK). The fisheries use a minimum landing size (MLS) that stipulates the minimum size that may be harvested for commercial sale. However, in this paper, it states that because of the non-continuous growth of crabs due to moulting, it is very difficult to determine the age classes of crabs with the significant overlap in size and moult increments. Because of this overlap, Bateman et al., (2011) separated Cancer pagurus into two subgroups; those below the MLS termed as prerecruits, and crabs above the MLS termed as recruits. Most information published relates to recruits with almost nothing known about disease processes and mortality drivers in prerecruits.

It is considered in this paper that prerecruits should only be limited by natural mortalities caused by disease and/or predation, whereas recruits should be additionally limited by fishing mortality. Therefore, the intention of this study was to concentrate on factors that may limit the likelihood of prerecruit populations eventually being recruited to the fisheries with an overview aim to attempt to estimate recruitment success and future supply on a yearly basis. It is worth pointing out that additional work alongside this study would be needed as assumptions such as crab movement to different areas are just one implication. Amongst the pathogen profile of C. pagurus which mainly consists of profiles for adults (recruits), there have been some studies on the pathogens of juveniles (prerecruits) with examples ranging from viruses, metazoans, and parasitoids appearing to show a specific preference for infecting younger age classes.

In an attempt to provide an overview of potential mortality drivers associated with pathogenic infections in prerecruit and recruit subpopulations of C. pagurus, Bateman et al., sampled crabs from the English Channel, UK (Weymouth Bay). Prerecruit crabs were sampled from the shore and recruits (above MLS, > 14 cm) from commercial fishing vessels operating immediately offshore of the shore-collection sites. Samples were collected each month for 12 months and analysed using histology and electron microscopy for the detection and identification of pathogens. The hepatopancreas, gill, gonad, heart, antennal gland, epithelial tissue, body muscle, and claw muscle were removed from each crab for histology. Fixed for twenty-four hours; samples were processed to wax in a vacuum in filtration processor using standard protocols. Sections were cut at a thickness of 3–5 mm and mounted on glass slides before staining. Stained sections were analysed by light microscopy digital imagery and measurements were taken. For electron microscopy, the hepatopancreas, antennal gland, epithelial tissue, and gonad were removed and fixed. Fixed tissue samples were rinsed, and then post-fixed for one hour. Specimens were washed before dehydration and then embedded in epoxy resin and polymerized overnight at 60 °C in an oven. Semi-thin (1–2 mm) sections were stained for viewing under a light microscope to identify suitable target areas. Ultra-thin sections (70–90 nm) of these areas were mounted on uncoated copper grids and stained with the grids being examined using a transmission electron microscope.

The pathogen profile and apparent prevalence of specific pathogens varied with season and, significantly, differed between prerecruit and recruit subpopulations. Three pathogens predominated in the histopathological analyses carried out on recruit and prerecruit crab subpopulations;  Hematodinium sp., which causes so-called pink crab disease, was detected in both prerecruit and recruit subpopulations  but apparent prevalence was higher in recruits (peaking in spring/summer months and with a smaller peak over the winter). The highest infection in prerecruits was in August, but in contrast to the recruit subpopulation, did not appear as a distinctive peak, but rather as a low-level prevalence in most months of the year. The two remaining high apparent prevalence pathogens appeared to be limited exclusively to prerecruits; Microphallus primas and a novel and highly prevalent haplosporidian-like parasite was also discovered infecting the antennal gland and bladder of the prerecruit crabs. Many pathogens were observed at relatively low prevalence in subpopulations of recruit and prerecruit C. pagurus. Usually, the prevalence of these pathogens did not appear to indicate any obvious seasonality, although, in general, the pathogenic expression of disease caused by these infections was severe and apparently led to a significant negative effect on the fitness of individual host crabs.

With increasing pressure on global commercial stocks of crustaceans, there is a growing requirement to understand the factors that limit recruitment to the fishery and, further, to understand so-called silent mortality drivers in the prerecruit subpopulation. Disease assessments in shoreline (prerecruit) populations of C. pagurus are proposed as a means of forecasting likely future recruitment to the commercial fishery for this species and, as such, might be a useful tool for predicting long-term stability of the fishery. It is understandable that juvenile crabs 'disappear' to grow via moulting but I believe that variation in when the crabs were collected (recruits; December 2003 & November 2004, prerecruits; April 2008 & March 2009) may have an impact on the results. Depending on the season, pathogens can vary in type and abundance, however, the key finding of this study does highlight the fact that the prerecruit crab population appears to be susceptible to a different and greater range of pathogens and parasites than the recruit subpopulation.

Bateman, K.S., Hicks, R.J., & Stentiford, G.D. (2011) 'Disease profiles differ between non-fished and fished populations of edible crab (Cancer pagurus) from a major commercial fishery' ICES Journal of Marine Science 68(10): 2044-2052.