Summary 1. Fisheries models generally are based on the concept that strong density dependence exists in fish populations. Nonetheless, there are few examples of long‐term density dependence in fish populations. 2. Using an information theoretical approach (AIC) with regression analyses, we examined the explanatory power of density dependence, flow and water temperature on the per capita rate of change and growth (annual mean total length) for the whole population, adults, 1+ and young‐of‐the‐year (YOY) brook trout (Salvelinus fontinalis) in Hunt Creek, Michigan, USA, between 1951 and 2001. This time series represents one of the longest quantitative population data sets for fishes. 3. Our analysis included four data sets: (i) Pooled (1951–2001), (ii) Fished (1951–65), (iii) Unfished (1966–2001) and (iv) Temperature (1982–2001). 4. Principle component analyses of winter flow data identified a gradient between years with high mean daily winter flows, high daily maximum and minimum flows and frequent high flow events, and years with an opposite set of flow characteristics. Flows were lower during the Fished Period than during the Unfished Period. Winter temperature analyses elucidated a gradient between warm mean, warm minimum and maximum daily stream temperatures and a high number of minimum daily temperatures >6.1 °C, and years with the opposite characteristics. Summer temperature analyses contrasted years with warm summer stream temperatures vs years with cool summer stream temperatures. 5. Both YOY and adult densities varied several‐fold during the study. Regression analysis did not detect a significant linear or nonlinear stock–recruitment relationship. AIC analysis indicated that density dependence was present in 15 of 16 cases (four population segments × four data sets) for both per capita rate of increase (wi values 0.46–1.00) and growth data (wi values 0.28–0.99). The almost ubiquitous presence of density dependence in both population and growth data is concordant with results from other trout populations and other studies in Michigan.

Summary We used long‐term population data for rosyside dace (Clinostomus funduloides), a numerically dominant member of a stochastically organised fish assemblage, to evaluate the relative importance of density‐dependent and density‐independent processes to population persistence. We also evaluated the potential impacts of global climate change (GCC) on this species and predicted how directional environmental changes will affect dace. We sampled two 30 m permanent sites in spring and autumn in the Coweeta catchment for rosyside dace density using three‐pass electrofishing between 1984 and 1995, and a single 100 m site from 1991 to 2003. Habitat availability and flow variation data for this 20‐year period demonstrated that two droughts (1985–1988 and 1999–2002) produced smaller wetted areas, lower mean, maximum and minimum flows, fewer high flow events and greater amounts of depositional substrata in the sites. Droughts produced significant increases in abundance, and significant decreases in standard length and mass of rosyside dace. Increases in abundance were mainly due to increased survival/immigration of young‐of‐the‐year (YOY). Model selection analysis using multiple single and multivariable models indicated that density dependence in various forms possessed substantial explanatory power with respect to long‐term variation in the per‐capita rate of increase (r) in all sites and seasons. Density‐dependent effects on r were stronger in autumn than spring, whereas negative density‐independent models (flow variation) had the greatest explanatory power in spring. Results for growth data were similar to those for rosyside dace density and confirm density dependence likely through intraspecific competition for food or foraging sites leading to reduced growth at higher densities. These data support the hypothesis that species may persist in stochastic animal assemblages via strong intraspecific density dependence. Greater flow variability or increased high flows produced by GCC may destabilise this population leading to reduced compensation and possibly eventual extinction.

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For animals that develop externally, habitats where environmental conditions are optimal for embryonic development are sometimes assumed to represent the highest recruitment potential and thus support the majority of reproductive output for a species. However, organisms may spawn in areas considered sub-optimal for embryonic development. Thus, understanding spawning habitat selection decisions and their potential impacts on recruitment and ecological interactions is necessary for predicting population status and identifying critical habitats to inform sustainable conservation decisions and effective management approaches. The American horseshoe crab, Limulus polyphemus, is ecologically, economically, and biomedically important. Females come ashore to spawn in the sediment where eggs develop for 2 – 4 weeks. Horseshoe crabs have been thought to primarily use sandy beach habitat for spawning in part because this habitat has been shown to be optimal for embryonic development. Horseshoe crab eggs on sandy beaches are an essential part of the diet of many organisms, including shorebirds such as the rufa red knot which requires the eggs to fuel their migration to arctic spawning grounds. While horseshoe crabs have been observed spawning in alternative habitats such as salt marshes and peat beds, this behavior has been assumed to be rare and non-adaptive. In this study, we compare the use of beach and alternative habitats by horseshoe crabs for spawning. To do so, we conducted adult horseshoe crab spawning surveys and horseshoe crab egg surveys in beach and Spartina-dominated salt marsh alternative habitats in South Carolina, Connecticut, and New Hampshire, U.S.A. While spawning horseshoe crabs were more likely to be observed on beach habitats than in alternative habitats, potentially due to logistical constraints surveying alternative habitats, we found similar densities of spawning horseshoe crabs in both habitat types. We also tended to find more eggs in alternative habitats than on beaches. Taken together, these results suggest that alternative habitats likely represent a significant source of horseshoe crab spawning activity and recruitment that had not previously been quantified. We recommend this information be incorporated into horseshoe crab population assessments, habitat protections, and more directed research at understanding variability in habitat-specific horseshoe crab spawning and its relationship to migratory shorebirds.