Unexpected high discard mortalities of juvenile brown shrimp (Crangon crangon) in the North Sea shrimp fishery

The brown shrimp Crangon crangon is the target species of one of the largest crustacean fisheries in the North Sea, producing considerable amounts of bycatch every season. Mortality rates of bycaught juveniles have traditionally been considered negligible, but this assumption was based on studies with short haul durations and short observation times. We investigated shrimp mortality due to trawling and handling under conditions typical of the commercial shrimp fishery in the German Bight. A total of 1440 juvenile shrimps from 48 hauls with commercial beam trawls was analyzed in discard survival experiments, yielding insights into the effects of haul duration and two prevalent sieving methods (riddle and drum) on mortality rates. Shrimps were observed daily for three weeks to test for delayed mortality and moult performance as an indicator of fitness. On average, ca. 70% of the juvenile shrimps moulted successfully during the observation period, most of them (44.5%) in the first week. On average 6% of the shrimp were already dead in the pit. Mortalities of those transferred alive into the aquarium without passing the sieve reached mortalities over three weeks of up to 22% (June) in the 120 min hauls leading to a total mortality of 28%. Passage though a sieve resulted in a 67% (riddle sieve) or 100% (drum sieve) increase of mortality compared to the level that was observed from samples from the pit before sieving. In May 2016, a special situation was encountered with very high immediate (7.0–35.0%) and increased longer-term (19.4–31.9%) mortalities of juvenile shrimps regardless of treatment, indicating an unknown source of additional mortality. We hypothesize that these higher mortalities may be explained by undersized shrimp experiencing a repeated passage of the catch and sorting process of different vessels. In fact, our study emphasizes that bycatch mortalities of undersized shrimps estimated in earlier studies have been severely underestimated.