Applicability of different pot escape gaps to reduce bycatch of undersized snow crab (Chionoecetes opilio) based on laboratory observations

In snow crab ( Chionoecetes opilio ) fisheries, reduction of undersized and juvenile crab bycatch is important to avoid injuries and unintended crab mortality and support sustainable resource management, aiming at conserving fisheries resources and reducing catch sorting time onboard vessels. Conical pots are commonly used for capturing snow crab in different fisheries globally. When using such fishing gear, size selection usually takes place through pot netting meshes during the soak period. However, conical pots are often associated with bycatch of juvenile snow crabs. Earlier studies have attempted to evaluate whether additional size sorting mechanisms, such as escape gaps, with a rigid and more well-defined shape, could improve size selectivity in these fisheries. Understanding the behaviour of snow crab in response to these escape gaps is important as it can have an impact on escape success, number of attempts, and the time it takes for a successful passage through a specific escape mechanism. This study used a laboratory approach by filming movements of snow crabs of different sizes through different size and shapes of potential escape gaps and compared these movement patterns with those of pot netting mesh. By linking behavioural responses with morphological size constraints, the results showed that escape success strongly depended on crab orientation and that the escape gap design strongly influenced size selectivity. These findings highlight how behaviour–morphology interactions can inform escape-mechanism design in other crustacean fisheries, although such effects are expected to be species- and fishery-specific. The results confirmed that escape probability decreased with crab size, but that the time required for a successful escape attempt can be reduced when using escape gaps compared to netting meshes. • Behavioural observations are crucial to understand snow crab size selectivity. • Escape time and success depend on crab orientation and repeated attempts. • Circular gaps can optimally retain target crabs while releasing undersized crabs. • Elongated or wide gaps increase the risk of escape of target-sized crabs. • Side-by-side gaps or meshes can cause limb entanglement and delayed escapes.