Spatial variation in abundance and catch composition of Cancer pagurus in Norwegian waters: biological reasoning and implications for assessment

Abstract

The edible crab (Cancer pagurus), which is abundant along the Norwegian coast north to Troms County, has been exploited in Norway since the start of the 20th century. The main fishery is in Mid-Norway and Helgeland (63°–67°N), which together land 75% of the Norwegian catch. The fishery is regulated by season, minimum legal size, and the discarding of soft or ovigerous females. Catches have never been systematically described, so this study presents data collected over 4 years by a reference fleet of professional fishers reporting the catch from four standardized trial traps set among their ordinary traps. Catch rate, catch composition, and size distribution from Mid-Norway/Helgeland are compared with the smaller crab fishery in Rogaland (59°N) and with new crab grounds off Vesterålen (69°N). Local variations in size and sex composition between and within region seemed partly to be caused by differences between exposed and sheltered regions, and between heavily and newly exploited grounds. Size distribution is used as an indicator to determine a suitable sampling strategy. The most efficient survey design is seemingly the collection of relatively small samples from many boats to reduce the between-vessel component of variation. A suggested strategy to obtain an abundance index would be to collect daily catch rates from all commercial landings.

Predators of the destructive sea urchin Strongylocentrotus droebachiensis on the Norwegian coast

“…Crab stocks have also increased tremendously within the area of kelp recovery (H. Christie pers. obs., and information provided by local fishermen) as indicated by the annual landings of C. pagurus which have tripled in central Norway during the last decade (Woll et al 2006) The whelk Buccinum undatum was also observed interacting with the sea urchins during our tethering experiments. It is difficult to evaluate whether B. undatum exerts a genuine predation pressure on sea urchins or whether the observed interaction simply was an artifact of the puncture method and the tethering procedure.…”

“…Climate change has great impact on species distributions, and the ongoing warming of the sea has proven to affect sea urchin−predator dynamics in both the northern and southern hemisphere (Ling & Johnson 2012, Clemente et al 2013). The range distribution of C. pagurus has recently expanded farther north in the NE Atlantic in correspondence with increasing ocean temperatures (Woll et al 2006, Brattegard 2011. Crab stocks have also increased tremendously within the area of kelp recovery (H. Christie pers.…”

“…A national report recently stated that more than 500 benthic marine species have extended their distribution farther north in Norway over the last 2 decades (Brattegard 2011). The ongoing recovery of kelp forests in central Norway coincides with large-scale changes in the abundance of Cancer pagurus (Woll et al 2006). Close relatives of this species have been identified as important predators of sea urchins in the NW Atlantic (Scheibling & Hamm 1991, Siddon & Witman 2004, McKay & Heck 2008, Steneck et al 2013) and the Pacific coast of North America (Clemente et al 2013).…”

“…The precipitous decline in the population of green sea urchins Strongylocentrotus droebachiensis O. F. Müller, 1776 seems related to a failure in recruitment, which has coincided with increasing ocean temperatures (Fagerli et al 2013). Throughout this period of increasing water temperatures and kelp recovery, an abrupt increase in the abundance of the edible crab Cancer pagurus Linnaeus, 1758 has taken place (Woll et al 2006). As a likely sea urchin predator, the increase in C. pagurus may contribute to the on-going process of kelp recovery.…”

Do bioturbation and consumption affect coastal Arctic marine soft-bottom communities?

“…Rising temperatures cause a decrease in physical disturbance by ice and a northward expansion and higher abundances of many boreal species (Weslawski et al 2011 and references therein). For example, benthic predatory crabs such as Cancer pagurus and H. araneus show increasing densities at the Norwegian and western Svalbard coast, respectively (Woll et al 2006;Berge et al 2009;Fagerli et al 2014). Due to higher consumer occurrence, we expect that new predator-prey interactions will develop in the future.…”