Missing pieces in the full annual cycle of fish ecology: a systematic review of the phenology of freshwater fish research

In recent decades, fish ecologists have become increasingly aware of the need for spatially comprehensive sampling. However, a corresponding reflection on the temporal aspects of research has been lacking. We quantified the seasonal timing and extent of freshwater fish research. Since reviewing all prior work was not feasible, we considered two different subsets. First, we compiled the last 30 years of ecological research on juvenile Pacific salmon and trout (Oncorhynchus spp.) (n = 371 studies). In addition to the aggregate, we compared groups classified by subject matter. Next, to evaluate whether riverscape ecology has embraced space at the expense of time, we compiled research across taxa in which fish were enumerated in a spatially continuous fashion (n = 46). We found that ecological Oncorhynchus spp. research was biased towards summer (40% occurred during June-August) and the month of June in particular, at the expense of winter work (only 13% occurred during December-February). Riverscape studies were also biased toward summer (47% of studies) and against winter (11%). It was less common for studies to encompass multiple seasons (43% of ecological Oncorhynchus spp. studies and 54% of riverscape studies) and most were shorter than 4 months (73% of ecological Oncorhynchus spp. studies and 81% of riverscape studies). These temporal biases may cause researchers to overemphasize ecological phenomena observed during summer and limit our ability to recognize seasonal interactions such as carry-over effects or compensatory responses. Full year and winter studies likely hold valuable insights for conservation and management.

Gametogenesis of the Pacific salmon. 3. Comparative analysis of the state of gonads in juvenile Pacific salmon due to fertility formation

The ovarian condition was studied in juveniles of six species of the Pacific salmon of different ages, taken for research at four hatcheries, as well as captured in lakes and rivers in the Sakhalin Province and Kamchatka Territory. The formation of the older generation of germ cells, consisting of previellogenic oocytes, in females of the Pacific salmon ends at the age of 0+, in pink salmon, with a mass of about 0.2–0.3 g, in other species, with a mass of about 1–2 g. In all species, the replenishment of this generation ceases during the habitat of juvenile fish in fresh water. After the formation of the older generation of germ cells is completed and its number reaches a certain level characteristic of each fish species, two oogenesis processes, that are not externally related to each other, continue to be carried out in the ovaries. The first process is the growth of the older generation oocytes, which develop relatively synchronously, varying 1.5–2 times in diameter. The second process is the mitotic reproduction of the gonies, their entry into meiosis, and subsequent resorption at the stage of pachytene and early diplotene. The mitotic activity of the gonies is minimal in females of the pink salmon, and, in fact, it is not detected in the fish caught in the coast. In females of other species, a decrease in both mitotic activity and initiation of new meiotic cycles does not occur during the entire period of their habitat in fresh water.

Vertical Distribution of Juvenile Salmon in a Large Turbid River

We know very little about the vertical distribution of downstream-migrating juvenile Pacific salmon Oncorhynchus spp. in large rivers. It is important for project engineers and fisheries managers to understand the potential interactions of fishes with in-river hydrokinetic devices, which harness a river's energy by spinning a turbine to produce electrical current without damming or impounding water. Currently, several rural Alaskan communities are considering development projects for hydrokinetic devices, including projects in the Tanana River, near Nenana, Alaska. Therefore, the goal of this study was to determine the vertical distribution of juvenile Chinook Salmon Oncorhynchus tshawytscha, Coho Salmon Oncorhynchus kisutch, and Chum Salmon Oncorhynchus keta, in the Tanana River, at a site (bottom depth ∼ 8, channel width ∼ 150 m) where communities may deploy future hydrokinetic turbines. Using a suspended wingless fyke net system during diurnal periods (0800–1800 hours), we found juveniles of all three species at all depths of the water column and no significant differences in catch-per-unit-effort among four depth categories (surface, midwater, deep water, bottom water). The occurrence of juvenile salmon throughout the water column indicates that they may interact with hydrokinetic devices, regardless of their depth. Future research to more comprehensively characterize fish distribution patterns and describe the outcomes of fish–turbine interactions may inform practices aimed at reducing potentially deleterious impacts of hydrokinetic devices on juvenile Pacific salmon.