Improved size selection for the Norwegian Red King Crab fishery by modifying the cage design and soaking time

The red king crab (Paralithodes camtschaticus) is a valuable offshore fishery resource for communities in northern Norway. The fishery east of the 26th meridian is regulated by vessel quotas and a minimum landing size of 130 mm carapace length. It was performed using a bait canister, and a round escape hatch had to be installed. Nevertheless, the crabs caught usually contain a large proportion of crabs that do not meet the legal size. This means an increase in labor-intensive sorting, which can lead to crab injury, loose limbs, or delayed death. This consequence has a negative impact on the sustainability of fisheries. In this study, we examined the potential of two new pot designs and longer soaking times to reduce the catch of oversized crabs in a comparative field trial.

The study found that designs consisting of tarpaulin panels at the entrance of the tank (designed to prevent undersized individuals from gaining traction and entering) are not good at reducing oversized catches compared to commercial designs with escape vents currently in use. The second design is a baited lower cavity (which can be accessed through the pelvic floor through an escape opening) to facilitate easier escape for smaller crabs. If caught commercially, any crabs in the bottomless cavity are left on the seabed while transported. This design was found to have minimal impact on subsurface catches. However, increasing the soaking time of the commercial design currently in use from 1 day to 4 days greatly reduces the catch of crabs that are too small.

This effect may be related to the depletion of the bait over time, increasing the chances and motivation of secondary crabs to find and leave the jar through the obligatory escape opening. Laboratory-based behavioral observations using panel design suggest that entry depends on the orientation of the crab, which is not as successful as commercial designs when entering. However, crabs of all sizes are inadvertently folded on the panel to increase the probability of their entry. This may explain the lack of success of this design in field trials. We concluded that by using longer soaking times than fishermen typically use to ensure bait depletion, the catch of oversized crabs in the fishery will be reduced.

1. Introduction

The Norwegian Red King Crab (RKC) fishery was established in 1994 as a research fishery and opened to commercial fishing in 2002 (Hjelset, 2014). There are quotas east of 26°E and south of 71°30'N, and free fishing in the west (Lovdata, 2022). The goal of regulatory areas is to maintain sustainable long-term RKC fisheries, while free fishing zones are intended to minimize population expansion to the west (Sundet et al., 2019). To ensure recruitment within the prescribed area, both male and female RKCs have a minimum landing size (MLS) of 130 mm nail length. All crabs caught outside the regulatory area, large or small, must go ashore. In 2021, quotas were set at 1629 tons for men, 120 tons for women and 181 tons for men (Lovdata, 2022). Despite its small numbers, the RKC's high individual value means that the species is economically important for coastal communities engaged in regulated fishing. Norwegian fishermen are targeting the RKC with collapsible single-chamber bait canisters. The maximum number of tanks per container is 30 (Lovdata, 2022).

Escape vents are rigid structures incorporated in pots that allow non-target species to escape (Saunders, 2009). They are known to reduce the number of undersized RKCs caught in the jar (Salthaug and Furevik, 2004, Jørgensen et al., 2017), and when fishing in the prescribed area, at least four circular escape rings (Å: ≥150 mm) must be installed on the walls of the tank. Nevertheless, catches of up to 80% of sublegal sized crabs are common (Siikavuopio et al., 2018). MLS legislation means that the crabs must be sorted on board and released into the sea. This can be labor-intensive and time-consuming for fishermen. In addition, RKC may lose limbs or be damaged due to crushing injuries during capture and sorting. In recent years, about 20 percent of RKCs have lost a leg or paw in regulated areas (A.M. Hjelset, Marine Institute, Personal Communications).

In the Barents Sea region of Russia, this proportion > 40%, and fisheries interactions are considered to be the main factor (Dvoretsky and Dvoretsky, 2009). Regeneration of lost limbs takes at least 7 years (Morado et al., 2014), and damaged crabs have lower growth rates, lower meat content, lower fecundity, and higher natural mortality (Dvoretsky & Dvoretsky, 2009, Stevens, 2014). Experiments in Alaska have shown that up to 20% of crabs that are treated and released may eventually die (Stevens, 2014). Given the potential impact on animal welfare, sustainability and profitability, many stakeholders are looking to reduce the catch of undersized RKCs in Norwegian fisheries. Since capture-related stressors are cumulative (Breen et al., 2020), the best solution is to avoid catching undersized RKCs as early as possible, preferably at fishing depths.

In order to achieve high fishing efficiency, fishermen usually use large amounts of bait and haul fish tanks every day. The attractiveness of the bait decreases over time as the odor is washed away and eaten by captive animals (Løkkeborg, 1990, Lørkeborg et al., 2014, Siikavuopio et al., 2017). Therefore, the selective property of the bait tank may not be fully released until it has been soaked long enough to deplete the bait, so the captured animal will have an incentive to escape. In Alaska, an increase in soaking time reduced the ratio between legal and legal RKCs caught in baited jars (Pengilly and Tracy, 1998).

Therefore, by increasing the soaking time, it is possible to reduce the oversized catch in the Norwegian RKC tank. Another way to reduce undersized catches is to try to prevent these individuals from entering the pond in the first place. This can be achieved by changing the design of the jar. In Canada, the introduction of "groups" that are difficult for a small individual to pass has reduced the catch of undersized snow crabs (Chionoecetes opilio) (Chiasson et al., 1993).

In this study, we developed two new tank designs designed to reduce catches of undersized RKCs and tested their size-selective properties through a series of comparative fishing trials in northern Norway. These designs are based on the principle that illegal captures can be reduced by discouraging these people from entering, or by incentivizing them to leave after entering. We also looked at whether the size selection property could improve the pot design for current commercial use by using longer steeping times.

Since the catch efficiency of passive gear with bait, such as fishing gear, depends largely on the behavioral response of the target species (Johannessen et al., 1993, Anders et al., 2017), behavioral studies can help explain the performance of different gear designs during fishing trials, thus proposing improved methods (Fernö, 1993). Therefore, we also conducted laboratory observations of RKC behavior while interacting with our new design.

The overall goal of our study is to determine which tank design or salvage strategy (i.e., increase soak time) optimizes the size selection of RKC tanks compared to designs currently used commercially on property. Selective property of fishing gear is optimal if individuals below legal size are avoided while maintaining or increasing the catch of target individuals. Specifically, we aim to address the following research questions:

(i) Can the RKC tank catcher be optimized by setting a "panel" around the inlet to limit the design of its inlet?

(ii) Can RKC tank catches be optimized to improve escape rates by designing them to include bait "escape chambers"?

(iii) Can observations of RRKC behavior in the lab explain the newly designed size selection property?

(iv) Is it possible to optimize the capture of the RKC tank by extending the soaking time?

Improving size selection in the Norwegian red king crab (Paralithodes camtschaticus) fishery through modification to pot design and soak time.