Predation mortality from ambush and cruising predators on newly-settled 0-group gadoids

Abstract

We investigated predation from an ambush predator (shorthorn sculpin) and two cruising predators (Atlantic cod and saithe) on bottom-settled 0-group cod and saithe juveniles in a fjord in northern Norway. Stomach contents of potential fish predators caught by gill net in shallow waters (depth of 3–10 m) during September in 2013–2015 were analysed. The frequency of occurrence (FO) of 0-group gadoid prey was highest for shorthorn sculpin (FO = 10%) and cod (FO = 9%), while it was lower for saithe (FO = 0.8%). Each predatory fish species had different alternative prey to 0-group gadoids: shrimps for cod; sandeels for saithe; and Brachura/Anomura for shorthorn sculpin. Amongst the fish predators, shorthorn sculpin had the highest catch rates in gill nets. The mean number of 0-group cod and saithe in shorthorn sculpin stomachs increased with increasing abundance of prey as measured by beach seine hauls.

Predation mortality from shorthorn sculpin was estimated based on abundance of shorthorn sculpins, number of 0-group cod and saithe juveniles in predator stomachs, prey recognition times and abundance of 0-group cod and saithe. The average predation mortality rate from shorthorn sculpin on 0-group gadoid juveniles varied from 0.40 to 1.14 month−1 between years and the pooled estimate from 2013 to 2015 was 0.70 month−1 (95%CI: 0.30, 1.73). This is comparable to mortality rates from other studies on newly-settled 0-group Atlantic cod, other gadoid species and plaice. Based on gill-net catches and stomach data from this study and literature values for gill-net catching area, calculated predation mortalities from the cruising cod and saithe were less than 47% of the sculpin predation mortality rates. This show that predation mortality from the ambush sculpin predator was much higher than from the cruising Atlantic cod and saithe.