Shore crab (Carcinus maenas) in inner Danish waters: A case study in the Lillebælt fishery

The European shore crab Carcinus maenas is native to northeast Atlantic, but a significant invasive species elsewhere, inhabiting a wide range of habitats in coastal waters due to its extraordinary phenotypic plasticity, environmental tolerance and omnivorous diet.

Qualitative data on shore crab populations is very limited or non-existent, but it is generally accepted that populations in inland and coastal Danish waters are very large and/or increasing. Shore crab fishing in Denmark thus has the potential to expand and become a viable and sustainable exploited resource. In this study, the shore crab pot fishery in the Lillebælt was monitored monthly to bimonthly between May 2023 to September 2024 to assess seasonal variation in catches and its composition, and fish bycatch. In addition, the short-term fishing impact on a local shore crab population (ca. 0.14 km2) was evaluated from two 2-week intense fishing trials in early July and September 2024.

Shore crab catch rates were highest in late spring and summer (> 100 crabs/pot/day) and lowest in winter (< 10 crabs/pot/day), and spatially variable between sites. Catch rates increased weakly with water temperature but were unaffected by water depth or the nearby presence of eelgrass. Male crabs dominated catches and the predominance of the red and green colour morphotypes varied with season and sex. Fish bycatch varied seasonally, being mainly wrasses (62%), gobies (19%) and juvenile cod (10%).

Impact from intensive fishing was limited to shallow locations where the coastline imposed a boundary limiting migration of crabs, while in deeper further offshore locations catch rates were unchanged. Lower catch rates in the second trial followed the seasonal decrease in catches. Catches were male dominated, but spatially variable with female abundance increasing in shallow locations. No decreasing trend in crab size was observed during the trials, with migration explaining larger females and smaller males observed in the second trial. Even though 7,800 and 4,840 kg and ca. 250,000 and 170,000 shore crabs were caught from a small area in the two week trials, the local shore crab population showed significant resilience and recovery ability to short term impacts from intensive fishing.