There is increasing recognition of intraspecific diversity and population structure within marine fish species, yet there is little direct evidence of the isolating mechanisms that maintain it or documentation of its ecological extent. We analyzed depth and temperature histories collected by electronic data storage tags retrieved from 104 Atlantic cod at liberty ≥1 year to evaluate a possible isolating mechanisms maintaining population structure within the Icelandic cod stock. This stock consists of two distinct behavioral types, resident coastal cod and migratory frontal cod, each occurring within two geographically distinct populations. Despite being captured together on the same spawning grounds, we show the behavioral types seem reproductively isolated by fine-scale differences in spawning habitat selection, primarily depth. Additionally, the different groups occupied distinct seasonal thermal and bathymetric niches that generally demonstrated low levels of overlap throughout the year. Our results indicate that isolating mechanisms, such as differential habitat selection during spawning, might contribute to maintaining diversity and fine-scale population structure in broadcast-spawning marine fishes.

In exploited fish stocks, long-term trends towards earlier maturation have been interpreted as an evolutionary response to sustained, high fishing mortality. The evidence used to support this diagnosis consists of directional shifts in probabilistic maturation reaction norms (PMRNs) that are consistent with the expectation that high fishing mortality favours the genotype for early maturation. Most PMRNs describe the probability of becoming mature solely as a function of age and length. Because they do not fully account for several physiological aspects of maturation (including growth effects on maturation, critical time windows for maturation decisions and developmental thresholds for maturation), it is possible that the observed shifts in PMRNs that are currently being attributed to changes in genotype actually reflect environmental effects on maturation. In this study, a comparative approach was used to interrogate the historical database for Northeast Arctic cod Gadus morhua in relation to 2 contrasting but not mutually exclusive hypotheses: (1) that there is a significant effect of food availability on the probability of being mature, using condition as a proxy for food availability; and (2) that there has been a long-term shift in the PMRN for maturation in a direction that is consistent with a diagnosis of fisheries-induced evolution. The results show that the maturation trends in Northeast Arctic cod could be variously interpreted as showing a strong environmental effect, no genetic effect, or a strong genetic effect. If the scope of the analysis had been restricted to testing a single hypothesis related to either an environmental effect or a genetic effect, then the study could very easily have made a Type I error of inference. A more integrated view of maturation, incorporating key aspects of the physiological processes that culminate in maturation, is therefore required to avoid incorrect inferences about the underlying causes of earlier maturation.

Otolith shape can be used to identify ecotypes of the Icelandic cod (Gadus morhua) stock. The use of data storage tags has increased our knowledge of the stock structure of Icelandic cod. The profiles of tagged cod reveal different migratory strategies. This has led to the definition of two ecotypes within the cod stock. Frontal ecotypes reside in deep waters during feeding season and express a highly variable temperature profile associated with thermal fronts, while coastal ecotypes stay in shallow waters all year round. In this study, the data storage tag profiles were analysed with cluster analysis, which revealed the existence of an intermediate behaviour that expresses a variable depth profile and feeding migration that is both shorter in time and not as deep. The main objective was to develop a morphological key based on otoliths to distinguish the ecotypes. The shape of the otoliths was extracted with shape measurements and fast Fourier transforms. A discriminant function analysis indicated a difference in morphology between the ecotypes, resulting in successful classification.

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