The biology and exploitation of three crab species in the Shetland Islands, Scotland: Cancer pagurus, Necora puber and Carcinus maenas

Tallack, Shelly M. L. (2002). The biology and exploitation of three crab species in the Shetland Islands, Scotland: Cancer pagurus, Necora puber and Carcinus maenas. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f702

Abstract

The Shetland crab fishery represents an important component of the local inshore fishing industry. Intensification of fishing practices and the recently implemented local management tool - The Shetland Islands Regulated Fishery (Scotland) Order 1999 - have highlighted the need for biological and fishery information on the two key commercial crab species, Cancer pagurus (L.) and Necora puber (L.). Findings made available to the Shetland Shellfish Management Organisation should enable local legislation to incorporate tailored and effective management measures to ensure the sustainability of Shetland's crab stocks.

C. pagurus sampled across a wide size-range (3-237mm CW), facilitated estimates of growth parameters as follows: K=0.188, L=234.3mm CW (males) and K=0.224, L=212.2mm CW (females); size at maturity estimates were 116mm CW (males) and 128mm CW (females). Mean brood fecundity ranged from 771,485 - 2,433,758 eggs. Natural mortality estimates of M=0.242 (males) and M=0.256 (females) were obtained. Both limb-loss (LL) and black spot disease (BSp) were more prevalent in males (27.7% & 26.0%) than in females (20.7% & 11.8%), with BSp occurring more frequently in injured crabs; prevalence of both LL and BSp increased with increasing crab size.

N. puber were sampled across the size range of 3.5-109mm CW, but juveniles were rarely obtained. Growth parameter estimates were K=0.463, L=102.2mm CW (males) and K=0.461; L=96.4mm CW (females). Size at maturity estimates were 45mm CW (males) and 56mm CW (females); mean brood fecundity ranged from 162,528 - 199,364 eggs. Natural mortality estimates of M=0.576 were obtained for both males and females. Limbloss was more prevalent in males (15.9%) than in females (8.5%); cheliped reversal was recorded in 6.0% of the N. puber sampled.

Length-cohort analysis indicated increased population abundance for both C. pagurus and N. puber based on current fishing levels and the recently increased minimum legal sizes of 140mm and 70mm CW respectively. Length-cohort analysis and yield per recruit analysis in combination indicated a need for reducing fishing effort by approximately 30% in Shetland. Additional fishery issues requiring attention include handling techniques and the C. pagurus claw-only fishery.

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