Food resource utilization by juvenile Baltic cod Gadus morhua:a mechanism potentially influencing recruitment success at the demersal juvenile stage?

Abstract

Pelagic and demersal juvenile Baltic cod Gadus morhua L. were collected during surveys in the Bornholm Basin (Baltic Sea) in autumn 1994. Stomach contents were examined for prey composition in order to evaluate the potential importance of the pelagic and demersal habitats for recruitment success. Juvenile cod less than 40 mm fed exclusively on pelagic prey such as copepods and cladocerans. Between 40 and 50 mm the juveniles began to consume benthic prey such as mysids and amphipods; however, copepods were still the dominant food organisms. Between 50 and 70 mm the dominant prey items consumed were mysids and amphipods, with copepods comprising a minor component of the diet. Between 70 and 160 mm the juveniles' diet was composed exclusively of benthic prey with an increase in prey diversity. Mysids were slightly less important in the diet, while the abundance of polychaetes, decapods and fish increased with fish size. Amphipods remained almost constant in the diet of juveniles above 60 mm in length. These results indicate that a major change in food resource utilization started to occur at a fish length of 40 mm, with the main change taking place at approximately 50 mm, suggesting that juvenile Baltic cod make the transition to the benthic habitat at this length. This change in food preference is also reflected in the size of the prey items and the numbers of prey consumed. Pelagic juveniles consumed much smaller prey than their demersal conspecifics. Neither prey size nor prey numbers consumed by pelagic and small demersal juveniles from this study differed from prey reported to be taken by juvenile cod of corresponding size in the Atlantic and the North Sea. However, prey organisms consumed by demersal juvenile Baltic cod larger than 90 mm were much smaller, and prey numbers much higher, than those reported to be consumed by juveniles in other areas. The apparent overlap in food resource utilization among the different size groups of demersal juveniles observed in this study suggests that in years with low prey abundance, or high abundance of pelagic juveniles, strong intra-specific competition for food resources may affect the recruitment success of Baltic cod.

Using otolith microstructure to analyse growth of juvenile Baltic cod Gadus morhua

“…The diversity of potential prey species in the Baltic Sea is limited, forcing newly settled cod to feed on the same size-range and species of prey as fish of a larger size (Hüssy et al 1997). The major prey items during the first months of the demersal stage are benthic and semi-benthic invertebrates such as mysids and amphipods (Hüssy et al 1997), whose highest densities are found below 25 m (Aschan 1988, Rudstam & Hansson 1990).…”

“…The major prey items during the first months of the demersal stage are benthic and semi-benthic invertebrates such as mysids and amphipods (Hüssy et al 1997), whose highest densities are found below 25 m (Aschan 1988, Rudstam & Hansson 1990). On the slope, feeding conditions may therefore have been more favourable than on the shallow bank due to greater variety and quantity of prey items, as suggested by Hüssy et al (1997).…”

“…The major prey items during the first months of the demersal stage are benthic and semi-benthic invertebrates such as mysids and amphipods (Hüssy et al 1997), whose highest densities are found below 25 m (Aschan 1988, Rudstam & Hansson 1990). On the slope, feeding conditions may therefore have been more favourable than on the shallow bank due to greater variety and quantity of prey items, as suggested by Hüssy et al (1997). Since the density of juvenile cod was also lower on the slope, and mortality due to predation is negligible (Uzars & Pliksh 2000), density-dependent competition for food may have been a mechanism responsible for the observed differences in growth.…”

Feasibility of removal of dioxin and dioxin-like PCB's by intensive fishery of herring and sprat in the Baltic Sea

“…Herring in particular are important consumers of a key prey item of the diets of larval and pelagic 0-group cod, Pseudocalanus spp. (Hussy et al 1997;Voss et al 2003;Möllmann et al 2004;. This copepod species is presently relatively rare in the Baltic sea due to both a multi-annual change in hydrographic conditions (i. e., lower salinity; and predation by herring [Casini, 2008 3715 /id;Möllmann, 2008 3716 /id;Casini et al 2009].…”

Foraging performance of juvenile Atlantic cod Gadus morhua and profitability of coastal habitats

“…Cod spend their first months of life in pelagic environments feeding on zooplankton. When reaching a sufficient size (approximately 5 cm), cod become benthivorous and commonly forage on various benthic crustaceans associated with vegetated habitats (Hüssy et al 1997). As cod grow in size, the proportion of fish such as herring and sprat in their diet increases.…”