Growth patterns of three cohorts of Atlantic cod larvae (Gadus morhua L.) studied in a macrocosm

Cross-frontal variation in growth rate and prey availability of larval North Sea cod Gadus morhua

“…15 mm are within the range of the present findings; e.g. a SGRW of 0.08 d -1 (at 5°C) for larvae at Georges Bank (Bolz & Lough 1988) and 0.04 to 0.085 d -1 (at 3 to 8°C) for larvae at Browns Bank , while SGRW estimates from mesocosm studies reach levels of 0.13 to 0.17 d -1 (6 to 12°C) (Folkvord et al 1994, Suthers et al 1999. Beside variation in prey availability, temperature differences between study areas influence attainable growth rates (Folkvord 2005).…”

Cannibalism of Atlantic cod Gadus morhua larvae and juveniles on first-week larvae

“…In the present study, the frequencies of aggressive and cannibalistic behaviours increased as the size differences between prey and predator increased. The age difference between prey and both the larger and smaller predators in our study was 48 and 34 d, respectively, and Folkvord et al (1994) showed that cannibalism could be a major cause of mortality when the age difference among cohorts is >10 to 20 d and the food supply is limited. Cannibalistic behaviour of larger conspecifics on first-week cod larvae appears to develop sometime between 6 and 9 mm.…”

“…Cannibalism studies on larval and early juveniles of cod received less attention, likely because the available field information rules out intra-cohort cannibalism (Ellertsen et al 1984, Perry & Neilson 1988. However, studies of semi-intensive systems, such as large marine ponds, suggest intra-cohort cannibalism in cod (Folkvord et al 1994, Blom & Folkvord 1997. Sakakura & Tsukamoto (1996) showed that young yellowtail Seriola quinqueradiata exhibit strong cannibalistic behaviour at 23 dph (10 mm total length), and they attributed this elevated aggression to greater size variation within the cohorts.…”

“…Folkvord et al (1994) reported that in situations where cohorts of Atlantic cod larvae separated by 20 d in age were reared together, mortality rates in the younger cohort were higher when copepod densities were low. At that time, the cohorts had standard lengths of 9 to 15 mm (older) and 5 to 7 mm (younger).…”

“…They suggested that the early juveniles from older cohorts had the ability (having the length ratio of 1.3:1 to 3:1) and motivation (low copepod density) to cannibalize the younger cohort. Folkvord et al (1994) therefore concluded that cannibalism was the most probable explanation for the high rates of mortality, rather than food availability or other unexplained environmental factors. Our results support their prediction, which showed that the attack frequencies increased significantly as the predator size increased from 12 to 15 mm.…”