Predator–prey reversal: A possible mechanism for ecosystem hysteresis in the North Sea?

Removal of large predatory fishes from marine ecosystems has resulted in persistent ecosystem shifts, with collapsed predator populations and super-abundant prey populations. One explanation for these shifts is reversals of predator–prey roles that generate internal feedbacks in the ecosystems. Pelagic forage fish are often predators and competitors to the young life stages of their larger fish predators. I show that cod recruitment in the North Sea has been negatively related to the spawning-stock biomass of herring for the last 44 years. Herring, together with the abundance of Calanus finmarchicus, the major food for cod larvae, were the main predictors of cod recruitment. These predictors were of equivalent importance, worked additively, and explained different parts of the dynamics in cod recruitment. I suggest that intensive harvesting of cod has released herring from predator control, and that a large population of herring suppresses cod recruitment through predation on eggs and larvae. This feedback mechanism can promote alternative stable states and therefore cause hysteresis to occur under changing conditions; however, harvesting of herring might at present prevent a shift in the ecosystem to a herring-dominated state.

Within-Otolith Variability in Chemical Fingerprints: Implications for Sampling Designs and Possible Environmental Interpretation

Largely used as a natural biological tag in studies of dispersal/connectivity of fish, otolith elemental fingerprinting is usually analyzed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). LA-ICP-MS produces an elemental fingerprint at a discrete time-point in the life of a fish and can generate data on within-otolith variability of that fingerprint. The presence of within-otolith variability has been previously acknowledged but not incorporated into experimental designs on the presumed, but untested, grounds of both its negligibility compared to among-otolith variability and of spatial autocorrelation among multiple ablations within an otolith. Here, using a hierarchical sampling design of spatial variation at multiple scales in otolith chemical fingerprints for two Mediterranean coastal fishes, we explore: 1) whether multiple ablations within an otolith can be used as independent replicates for significance tests among otoliths, and 2) the implications of incorporating within-otolith variability when assessing spatial variability in otolith chemistry at a hierarchy of spatial scales (different fish, from different sites, at different locations on the Apulian Adriatic coast). We find that multiple ablations along the same daily rings do not necessarily exhibit spatial dependency within the otolith and can be used to estimate residual variability in a hierarchical sampling design. Inclusion of within-otolith measurements reveals that individuals at the same site can show significant variability in elemental uptake. Within-otolith variability examined across the spatial hierarchy identifies differences between the two fish species investigated, and this finding leads to discussion of the potential for within-otolith variability to be used as a marker for fish exposure to stressful conditions. We also demonstrate that a 'cost'-optimal allocation of sampling effort should typically include some level of within-otolith replication in the experimental design. Our findings provide novel evidence to aid the design of future sampling programs and improve our general understanding of the mechanisms regulating elemental fingerprints.