Ranking of length-class seasonal and regional effects on dietary compositions of the co-occurring Pagrus auratus (Sparidae) and Pseudocaranx georgianus (Carangidae)

Using an effective combination of multivariate testing and ordination analyses, this study compares the extents to which the diets of two co-occurring fish species (Pagrus auratus and Pseudocaranx georgianus) are related to body size (length class), season and region and the rank order importance of those effects. Thus, volumetric dietary compositions were determined for these species on the lower west coast of Australia, where both are abundant, and for P. auratus from the mid west coast and P. georgianus from the south coast. The diet of P. auratus on the lower west coast was strongly related to body size and slightly less to season. With increasing body size, its diet shifted from predominantly ophiuroids to larger prey, such as brachyuran crabs, teleosts, echinoids and ultimately asteroids, probably reflecting a shift from foraging over soft sediments to areas over and around reefs. Seasonal changes on the lower west coast were restricted mainly to small P. auratus, while larger fish underwent seasonal changes further north. Analyses using a common size range of medium to larger P. auratus demonstrated that dietary composition differed more between regions than seasons. The relationships between diet and length class of P. georgianus on both the lower west and south coasts were less pronounced than for P. auratus and seasonal changes were restricted to the south coast, where amphipod consumption increased markedly in summer. The diet of P. georgianus was related far more to region than length class and season, with more small teleosts, small crabs, carideans and littorinids and less amphipods, isopods and small bivalves being ingested on the lower west than south coasts. Although crabs and teleosts were important typifying prey of P. auratus and P. georgianus, when co-occurring, the former predator tended to ingest greater volumes of larger and often less mobile prey. This reflects differences in dentition, jaw morphology and feeding behaviour and reduces the potential for competition for food resources. The results imply that P. auratus and P. georgianus are opportunistic feeders and that the effects of length class, season and region on dietary composition and their rank orders can vary markedly between species and for length class and season between regions for the same species.

Have climate induced changes in zooplankton communities led to poor conditions for larval fish growth? A test on cod larvae on the UK shelf

Climate effects have been shown to be at least partly responsible for the reorganisation in the plankton ecosystem on the shelf seas of NW Europe over the last 50 years. Most fish larvae feed primarily on zooplankton, so changes in zooplankton quantity, quality and seasonal timing have been hypothesized to be a key factor affecting their survival. To investigate this we have implemented a 1-dimensional trophodynamic growth model of cod larvae for the waters around the UK covering the period 1960 to 2003. Larval growth is modelled as the difference between the amount of food absorbed by the larva and its various metabolic costs. Prey availability is based upon the biomass and size of available preys (i.e. adults and nauplii copepods and cladocerans) taken from the Continuous Plankton Recorder dataset. Temperature and wind forcing are also taken into account. Results suggest that observed changes in plankton community structure may have had less impact than previously suggested. This is because changes in prey availability may be compensated for by increased temperatures resulting in little overall impact on potential larval growth. Stock recovery, at least in the short term is likely to be more dependent upon conserving the year classes recruited to allow spawning stock biomass to rebuild. If as our model suggests, the larvae are still able to survive in the changing environment, reduction in fishing on the adults is needed to allow the stock to recover.