Cod pots in a Baltic fishery: are they efficient and what affects their efficiency?

ICES Journal of Marine Science ICES Journal of Marine Science (2015), 72(5), 1545– 1554. doi:10.1093/icesjms/fsu230 Original Article Cod pots in a Baltic fishery: are they efficient and what affects their efficiency? 1 Department of Aquatic Resources, Institute of Coastal Research, SLU Swedish University of Agricultural Sciences, Turistgatan 5, Lysekil 453 21, Sweden Department of Aquatic Resources, Swedish University of Agricultural Sciences, Lundströms väg 1, Färjestaden 386 96, Sweden 3 Swedish Meteorological and Hydrological Institute, Oceanographic laboratory, Västra Frölunda, Sweden 4 Department of Aquatic Resources, Institute of Coastal Research, Swedish University of Agricultural Sciences, Skolgatan 6, Öregrund 74243, Sweden 2 *Corresponding author: tel: +46 702215915; e-mail: Königson, S. J., Fredriksson, R. E., Lunneryd, S-G., Strömberg, P., and Bergström, U. M. Cod pots in a Baltic fishery: are they efficient and what affects their efficiency? – ICES Journal of Marine Science, 72: 1545 – 1554. Received 20 November 2013; revised 20 November 2014; accepted 27 November 2014; advance access publication 11 January 2015. With the growing grey seal population in the Baltic Sea, the inshore cod fishery has suffered dramatic increases in both catch losses and damage to fishing gear. To mitigate this situation, cod pots were evaluated as an alternative to traditional gillnets and longlines. During a 3-year study, cod pots were used by commercial fishers in two areas off the coast of Sweden. Using the data from this study, we evaluated catches from pots in relation to other gear types and investigated the effects of environmental and fisheries-related variables such as depth and soak time. The comparison of pots with other gear types showed that, during the first half of the year, the pot fishery generated lower daily catches than the gillnet and longline fisheries at comparable fishing efforts. During the second half of the year, catches in the pot fishery exceeded or were equal to those in the traditional fisheries. Using generalized additive models to evaluate the impact of environmental and fisheries-related variables on pot catches, we showed that, in both areas, the catch per unit effort (cpue) of legal-sized cod was affected by the water depth, the time of year (months), and the soak time. In one of the areas, cpue was also affected by the direction of the water current in relation to the orientation of the string of pots. The cpue of undersized cod was affected by topographic variables such as the slope and the complexity of the bottom, in addition to the water depth, month of the year, and soak time. The results from the study indicate that pots can be a useful alternative gear in the Baltic cod fishery, at least during part of the year. By using our information on how catches are affected by environmental and fisheries-related variables, the pot fishery may be further optimized to increase catches. Keywords: catch per unit effort, catching efficiency, cod pot, environmental and fisheries-related effects, GAM, gillnet and longline fisheries. Introduction Seal-inflicted damage to fishing gear and catch losses have increased rapidly along the coast of the Baltic Sea, and many small-scale coastal fisheries have been severely affected (Westerberg et al., 2006; Bruckmeier and Höj Larsen, 2008; Hemmingsson et al., 2008). The gillnet and longline fisheries for Atlantic cod (Gadus morhua) have experienced extensive surges in damage caused by grey seals (Halichoerus grypus) since around the year 2000 (Königson et al., 2009). This has led to the development of seal-safe alternative fishing gear, such as baited cod pots. Cod pots are preferable to traditional methods in areas prone to seal predation, as they enclose the caught fish in a compartment which can be made seal-safe, meaning that it is much harder for the seals to get at the fish than when they are caught in a gillnet or hooked on a line (Königson, 2011; Ovegård et al., 2011). The environmental impact of traps and pots is also considered less severe than that of trawls and other active fishing gear (Jennings et al., 2001; Thomsen et al., 2010). Pots are classified as LIFE (low impact and fuel efficient) fishing gear due to their low # International Council for the Exploration of the Sea 2015. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/ licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Sara J. Königson 1 *, Ronny E. Fredriksson 2, Sven-Gunnar Lunneryd 1, Patrick Strömberg 3, and Ulf M. Bergström 4 1546 attractants from the bait are present in concentrations above the response threshold of the fish. This active space is determined by the release rate and transport of chemical cues through the water as well as the chemosensory threshold for an individual fish at a specific time (Løkkeborg et al., 1995). These components are, in turn, dependent on several of the above-mentioned environmental variables. Additionally, it is only a proportion of the fish in the active space that respond to bait or can be lured into the pot and be caught (Furevik, 1994; Løkkeborg, 1994; Kaimmer, 1999; Stoner, 2003). One aim of the present study was to evaluate environmental and fisheries-related variables affecting the catch in a pot fishery for cod in the Baltic Sea. In addition, we extrapolated catches to correspond to the effort that would be used in a commercial fishery, and compared these with catches from other commercial gear types in the same area. By simultaneously evaluating the influence of both fisheries-related and environmental variables, the relative effects of these factors can be separated, thereby aiding an understanding of factors determining the catching efficiency of the pot fisheries. Material and methods Experimental set-up Fishing with pots was conducted in collaboration with local commercial fishers in two areas along the Swedish coastline (Figure 1). In area 1, Hanö Bight, fishing trials were carried out over 2 years, from February 2009 until December 2010. In area 2, Karlskrona archipelago, fishing trials continued over 3 years, from 2009 until Figure 1. Map of the cod pot fishing areas in the Hanö bight. One dot on the map is the position of a string set during one fishing trip. The dots on the left side of the map are included in area 1 and the dots on the upper right are included in area 2. energy use, effective species selectivity, and low gear construction costs (Suuronen et al., 2012). At the time of writing, there is no production of commercial cod pots that meet the criteria of being practical to handle, seal-safe, and effective. Fishing gear manufacturers are in the process of further developing cod pots in collaboration with fisheries scientists. However, more knowledge of what affects the pots’ catching (...truncated)