Total and natural mortality of red king crab (Paralithodes camtschaticus) in Norwegian waters: catch–curve analysis and indirect estimation methods

ICES Journal of Marine Science ICES Journal of Marine Science (2015), 72(2), 642– 650. doi:10.1093/icesjms/fsu138 Original Article Total and natural mortality of red king crab (Paralithodes camtschaticus) in Norwegian waters: catch – curve analysis and indirect estimation methods Institute of Marine Research and Faculty of Biosciences, Fisheries and Economics, University of Tromsø, PO Box 6404, Tromsø NO-9294, Norway *Corresponding author: tel: +47 7760 9713; fax: +47 7760 9701; e-mail: Windsland, K. Total and natural mortality of red king crab (Paralithodes camtschaticus) in Norwegian waters: catch – curve analysis and indirect estimation methods. – ICES Journal of Marine Science, 72: 642 –650. Received 4 April 2014; revised 22 July 2014; accepted 23 July 2014; advance access publication 18 August 2014. The red king crab (Paralithodes camtschaticus) is native to the Bering Sea, but was deliberately introduced to the Barents Sea during the 1960s. Since then, the red king crab has spread to Norwegian waters, and crab densities have increased sufficiently to support a coastal fishery. Information about total and natural mortality, which is important to ensure adequate management, is lacking. Estimates of annual total mortality (Z) were calculated using length-converted catch curves in three periods of different levels of exploitation. Separate analyses were run on trap and trawl data as well as on original and catch per unit effort-corrected data. Natural mortality was estimated using a linear regression of total mortality and exploitation level and by using indirect methods based on life history parameters. There was a significant increase in Z throughout the consecutive periods in both sexes. In males, the increase coincided with the increase in exploitation level. The increase in female mortality, which was not explained by exploitation level, is probably an effect of the increase in male mortality. Natural mortality (M ), estimated using indirect methods, averaged 0.23 for both sexes. The estimated M using linear regression was 0.37/0.44, which may be an overestimate. Keywords: catch curve, decapoda, indirect methods, instantaneous natural mortality, instantaneous total mortality. Introduction The red king crab (Paralithodes camtschaticus) is native to the Bering Sea, but was deliberately introduced to the Barents Sea during the 1960s by USSR scientists (Orlov and Ivanov, 1978). The species is a valuable commercial resource in the Bering Sea, and the aim of the introduction was to benefit local fisheries. Since its introduction, the red king crab has spread to Norwegian waters, and densities have increased sufficiently to support a coastal fishery, with recent annual landings .2000 t (Hvingel et al., 2012). The commercial fishery started in 2002, and since 2004, there has been a steady decline in the population of legal males [carapace length (CL) ≥130 mm]. In 2012, stock abundance was considered to be relatively low (Anon., 2013a). To ensure adequate management of the stock, information is needed about instantaneous total mortality (Z) and natural mortality (M ). Catch–curve analysis can be used to estimate total mortality by observing the regular decline in the number of individuals in a cohort (Pauly, 1983, 1990). Age determination in crustaceans is difficult because hard parts are lost after ecdysis, and mortality assessments, therefore, must depend on length-based methods. By using an age–length key, ages can be assigned to an observed length in what is called a length-converted catch–curve (LCC) analysis. The slope of the catch curve, with sign changed, is a measure of Z for an exploited stock and a measure of M when exploitation is negligible. This method requires that the growth function used to convert length to age is representative of the entire sampling period. The use of cross-sectional analysis, using data from several years, further assumes that the data are representative of a stable age distribution, i.e. the samples used to construct the catch–curve should reflect average conditions during which recruitment has varied little or randomly (Pauly, 1987). Violations of this assumption may lead to catch curves that are non-linear in the descending right-hand limb. For the exploited stock, total mortality and average fishing effort over the same period can be used to estimate M. However, M is a difficult # International Council for the Exploration of the Sea 2014. All rights reserved. For Permissions, please email: Kristin Windsland* 643 Total and natural mortality of red king crab Material and methods Sampling area and data treatment The sampling area comprised four large fjords and open water off the coast of northern Norway (Figure 1). Stations were selected to provide coverage of all areas and depths. Data from the entire study area were pooled. For further information about sampling design, see Hvingel et al. (2012). In all, 64 535 red king crabs (30 323 females and 34 212 males) with carapace length (CL) ranging from 8 to 210 mm were caught during annual scientific cruises along the coast of northern Norway in August–November during 1994–2012. The crabs were collected using a large Agassiz trawl (n ¼ 26 634) and baited traps (n ¼ 37 902). CL of all individuals was measured to the nearest millimetre and sex was registered. In 1994, the most the population was resident in Varangerfjorden. During the sampling period, the distribution of red king crab has expanded to include Tanafjorden, Laksefjorden, and Porsangerfjorden (Figure 1). I separated the data by sex and gear (Agazziz trawl or traps) then divided it into 5-mm carapace length intervals. King crabs grow in a stepwise manner through moulting during specific periods of the year. The main moulting periods are January –April for males and April –May for females (Wallace et al., 1949; Powell, 1967). The red king crab in the southern Barents Sea seems to have the same moulting periods as in the Bering Sea (Nilssen and Sundet, 2006). The sampling period in all years took place between August and November when no growth in length occurs. Monthly samples were, therefore, pooled. The management of red king crab has changed during the period 1995–2012, and the sexes have also been subject to different levels of exploitation. Therefore, I ran separate analyses for males and females, and the dataset was divided into three periods: (i) 1995– 2001 (Period I, scientific fishery), (ii) 2002–2007 (Period II, early commercial fishery period), and (iii) 2008–2012 (Period III, late commercial fishery). In Period I, there was no commercial fishery on either sex and only a small scientific quota on males (Table 1). In Period II, there was a commercial quota (designated as number of animals) on males in addition to the scientific quota. Period III saw a significant increase in the male quota and the introduction of a female quota. In addition, the quotas were given in tonnes instead of number of animals. (...truncated)