Possible effects of different levels of krill fishing on predators – some initial modelling attempts
An initial attempt is made to develop the modelling framework suggested by the Joint Meeting of CCAMLR's Working Group on Krill (WG-Krill) and Working Group for the CCAMLR Ecosystem Monitoring Program (WG-CEMP) in 1992, in order to investigate the possible effects of krill fishing on krill predators. First, estimates are made of the parameters of predator survival rates as functions of krill abundance, by considering a krill dynamics model incorporating recruitment fluctuations together with preliminary information on adult survival and breeding success patterns for certain krill predator species. A 'one-way' interaction model is developed, in which krill abundance fluctuations impact on the predator population, but not vice versa. Computations based on this model indicate that variability in the annual recruitment of krill results in predator populations being less resilient to krill harvesting than deterministic evaluations would suggest. The analyses have also facilitated discussion on the interpretation of the preliminary estimates of adult survival rate provided for some krill predator populations, and on the possibility of biases in their estimation. These matters will need to be taken into account in future developments of this approach, before the latter can claim sufficient realism to provide a possible basis for management recommendations. A mathematical framework for a 'two-way' interaction model (including also the effect of differing predator consumption levels on krill) is developed, but computations based on this approach are deferred pending further development of the 'one-way' analyses.