Artificial stocking, an important tool in fishery resource enhancement and wildlife management, exerts negative effects on wild conspecifics. However, the factors determining the magnitude of these effects remain unclear. Here, we investigated how stocking influences genetic diversity, genetic structure, and effective number of breeders ( N b ) in Masu Salmon Oncorhynchus masou and how these effects vary with the stocking intensity, such as the number of fish stocked and proportion of non-native fish. We examined 15 microsatellites in 1989 individuals from 33 Masu Salmon populations and four hatchery stocks in Hokkaido, Japan. Stocking increased the allelic richness (Ar); however, this effect decreased with increasing stocking intensity. Lower genetic differentiation ( F ST ) was observed among populations affected by stocking than among native populations. N b correlated positively with the number of fish stocked and proportion of juveniles, but negatively with the proportion of non-native fish. Overall, this study shows that the genetic impact of stocking depends on various factors, such as the origin of stocked fish and stocking intensity.
Effective conservation of exploited species requires balancing harvest opportunities with the protection of biodiversity. In Pacific Salmon, biodiversity among populations contributes to basin-wide resilience, yet spatiotemporal dynamics of adult migrating Chinook Salmon ( Oncorhynchus tshawytscha) remain poorly understood in many large salmon-producing watersheds, creating uncertainty about management impacts. Declining abundance in western Alaska has led to restrictions in subsistence harvest and challenges for upstream communities. In the Kuskokwim River, a large salmon-producing river, a front-end closure limits early-season fishing to reduce exploitation of fish migrating to the upper watershed. To quantify stock-specific effects of this strategy, we reconstruct spatiotemporal dynamics of Chinook Salmon return timing across 6 years using otolith isotope signatures. We demonstrate a consistent relationship between seasonal run timing and watershed origin, with earlier migrants contributing disproportionately to upper watershed regions. A longer simulated closure provided greater protection for upper watershed stocks but with a tradeoff of lost harvest opportunity. Protection varied substantially among years due to migration timing, highlighting how fixed closure strategies interact with dynamic ecological processes to influence overall efficacy.
Freshwater fishes living in brackish environments often diverge into ecotypes displaying different habitat use. Salinity levels common to brackish habitats incur osmoregulatory costs to freshwater fishes, but also offer opportunities in the form of access to marine prey, creating trade-offs. We investigated foraging specialisation of coastal northern pike (Esox lucius) ecotypes, ranging from migratory to brackish- or freshwater- residents, sampled from coastal lagoons and freshwater tributaries in the southern Baltic Sea. We combined resource use with individual growth as a fitness surrogate to identify mechanisms of coexistence. Individual foraging specialisation was highest in behavioural phenotypes with high trophic overlap in shared foraging grounds, brackish resident pike and cross-habitat pike. Larger brackish-adapted individuals specialised more on marine resources with positive effects on growth, but higher-than average salinities reduced growth. Trophic overlap of anadromous and freshwater pike phenotypes was low and differed from cross-habitat and brackish habitat phenotypes. Age-specific effects of marine resources and salinity on individual growth suggest coexistence of pike ecotypes is facilitated by dynamic trade-offs between foraging specialisation and habitat costs.
Accurate age estimates of salmon produced from scale growth patterns yield demographic information essential for population assessment and fisheries management. Conventionally, salmon ages are estimated from scales by trained personnel where demand often exceeds staffing capacity and financial resources. Convolutional neural network (CNN) models are a promising approach to augment manual age estimation. We compiled an extensive scale image repository from chum salmon (Oncorhynchus keta, Salmonidae) stocks throughout the North Pacific rim, evaluated two image preprocessing algorithms—Segment Anything Model (SAM) and You Only Look Once (YOLO)—to remove or reduce background noise, and trained an ensemble of three CNN models (EfficientNetV2L, NASNetLarge, and ConvNeXtLarge, weighted by their MSE) to infer chum salmon age. Agreement with expert estimates was higher for the SAM preprocessed images (93.2%) than YOLO (91.4%) across all major age classes (ages 3–5). Both preprocessing approaches showed reduced agreement for less common age classes because of inadequate representation in the model training and validation datasets. This approach may be integrated into fisheries management workflows to accelerate the age estimation process, complement manual reading, and bolster quality control protocols.
Marine protected areas (MPAs) are widely implemented to protect or rebuild exploited populations, yet their population-level effectiveness remains variable. One unresolved question is how the spatial placement of no-take zones interacts with larval dispersal directionality, particularly in linear coastal systems where connectivity is often asymmetric. For sedentary species with planktonic larvae, such as European lobster, it remains unclear when reserve placement most strongly affects demographic recovery after exploitation. Here, we use a spatially explicit individual-based model parameterized for European lobster in a generic linear coastal system. We contrast Local, Broad, and Unidirectional larval dispersal scenarios across alternative MPA layouts with equal total protected area. We show that no-take zones increase abundance and the prevalence of large individuals relative to open areas. However, under Unidirectional dispersal, recovery depends on reserve placement: downstream no-take zones benefit from larval import and local retention, whereas upstream reserves primarily export larvae and show limited local recovery. These results identify dispersal directionality as a key determinant of when spatial configuration matters for MPA effectiveness.
