Partial migration in diadromous fishes drives the allocation of subsidies across the freshwater-marine ecotone

Migratory animals can act as cross-boundary subsidies sustaining ecosystem functioning, such as diadromous fishes that migrate between fresh water and seawater and carry nutrients and energy across the freshwater-marine ecotone. Frequency and timing of migration are however highly variable within and among populations. We hypothesized that in catadromous fishes (i.e., diadromous fishes that grow in freshwater and spawn in the sea, such as eels), the import of subsidies by migratory juveniles could outweigh the export of subsidies by adults due to skipped spawning migration. We used the diamond mullet Planiliza ordensis, as a model species, and determined life-history traits using a combination of length-to-age data, acoustic telemetry and otolith (fish ear stone) microchemistry. We used a mass balance approach to model individual mass acquisition and allocation, and extended our model to other life-history strategies. Our results showed high intra-population variation of migratory behaviour in P. ordensis, with few individuals migrating every year to spawn. We estimated that an individual P. ordensis acted as a net 42.6g biomass subsidy in fresh water, representing a retention of more than 50% of the juvenile mass at freshwater entry. Our model predicts that skipped spawning is likely to alter the allocation of subsidies in diadromous species, highlighting the important effects of individual variation in migratory behaviour on fluxes of energy and nutrient at ecosystem scales. We encourage future studies to consider how variation in migratory behaviour is likely to affect the direction and magnitude of biomass fluxes across ecotone boundaries.

Condition-dependent skipped spawning in anadromous brown trout (Salmo trutta)

Repeat spawners of anadromous salmonids may contribute significantly to population resilience by providing multiple cohorts to both seawater and freshwater life stages. In this study, winter survival of sea trout (Salmo trutta Linnaeus, 1758) post spawners (kelts) was 89%. Sea survival increased linearly with female length with a return probability between 30% and 50%, whereas males attained a maximum return probability of 60% at 520 mm. Of the returning sea trout, 40% skipped spawning and they had significantly lower condition factor as kelts compared with those who returned after one summer. These results suggest that sex-specific differences in individual post-spawning growth–survival trade-off exist and that energetic status of descending kelts may influence the probability to skip spawning. We discuss to what extent hydropower may reduce post-spawning survival due to blocking of river descent opportunities, potentially altering the fitness landscape and favouring new life-history adaptations. From a management perspective, it is concluded that it is crucial to maintain connectivity in regulated rivers, allowing fish to complete repetitive spawning and feeding migrations, thus contributing to population productivity.

Sex-specific differences in growth, mortality and migration support population resilience in the heavily exploited migratory marine teleost Mugil cephalus (Linnaeus 1758)

Understanding mechanisms supporting the resilience of exploited fish populations is fundamental to sustainable management. Herein we identify sex-specific differences in life history traits that confer resilience in the heavily exploited population of Mugil cephalus along eastern Australia. M. cephalus in this region emigrate from estuaries and undergo an annual northerly spawning migration that is dominated by males. Males mature, on average, at younger ages than females and were most abundant in the spawning migration at age-4, whereas females were most abundant at age-5. Females grew significantly faster and larger than males, with both sexes being aged up to 14 years. These sex-specific differences relate directly to the population stability of M. cephalus under the reproductive strategy of a spawning migration. Males participate in the spawning migration at younger ages and in greater numbers than females to ensure a sufficient number of males for successful spawning. Females partition more energy to growth than males, maturing and participating in the spawning migration at older ages and larger sizes. The larger body size of females is related to increased fecundity, increased survivorship and more efficient swimming ability. These traits, combined with population strategies of both delayed and skipped spawning, convey substantial resilience.