Potential fishing-related effects on fish life history revealed by otolith microchemistry

Although inferring habitat selection and movement in wild fishes using otolith microchemistry has been the subject of much debate, experimental results suggest that complex environmental vs. life history interactions affect the incorporation of certain chemical elements into otoliths. We hypothesized that under relatively homogeneous hydrographic conditions, individual life history trait strategies should dominate signals of geochemical signatures. We tested our hypothesis by analyzing individual chronologies of otolith signals of strontium to calcium (Sr:Ca) and barium to calcium (Ba:Ca) ratios in a sedentary coastal fish (Diplodus annularis). Two areas from the same northwest Mediterranean region with different levels of fishing pressure were sampled: (1) a no-take marine protected area where the "bigger-is-better" strategy should prevail (energy allocation is biased toward growth to compensate for high natural mortality at small sizes) and (2) a highly exploited population where the "smaller-is-better" strategy should dominate (energy allocation is biased toward reproduction to compensate for high fishing mortality at large sizes). Significant differences in reproduction vs. growth trade-offs were observed between the sub-populations. Sr:Ca and Ba:Ca chronologies were compared between back-calculated ontogenetic stages (larvae, juveniles and adults) and sub-populations/life histories using a general linear mixed model. The Sr:Ca chronology successfully reproduced the allocation of energy for growth or reproduction according to the life history strategy of the individual, whereas correlation between ontogeny and otolith microchemistry was less obvious for Ba:Ca. We contend that the direct and/or indirect (e.g., the removal of large predators) effects of fishing pressure on microchemical signals should receive more attention in field studies, as trade-offs between mortality and growth/reproduction might alter microchemical patterns under a given a set of conditions.