Quantifying euryhaline histories in red drum Sciaenops ocellatus: otolith chemistry and muscle isotope ratios.

The combined use of otolith chemistry and tissue isotopes has the potential to reveal movements, habitat associations and food web interactions at a variety of spatial and temporal scales. Here, we used a combination of otolith Ba:Ca life history transects with muscle tissue δ13 C and δ15 N values to assess habitat use and oligohaline residence in red drum Sciaenops ocellatus in subtropical estuaries in the northwestern Gulf of Mexico. Tissue isotopes were distinct among capture locations, particularly between bays with differing proximity to freshwater inflow sources. Otolith edge Ba:Ca values and tissue δ13 C values were not correlated. These results indicated that fish were not residing in nor feeding in oligohaline waters for significant periods of time within the tissue turnover window of several months prior to capture. However, spatial differences in tissue isotope values indicated limited mixing among bays and relatively high site fidelity during estuarine occupancy. Lifetime otolith Ba:Ca transects revealed individual variability in the magnitude of residence in oligohaline waters. Using a medium oligohaline occupancy threshold, an estimated 82% of individuals used oligohaline waters at some point in their life. However, 66% of individuals spent less than 20% of their life histories in oligohaline waters, suggesting intermittent and infrequent excursions into low salinity waters. Finally, a literature survey identified 56 peer-reviewed publications using combinations of otolith chemistry and tissue stable isotope ratios with a wide range of marker pairings and study aims. The diversity of ecological questions that can be asked with the combined use of these two approaches will provide valuable insight into fish ecology. This article is protected by copyright. All rights reserved.

Differences in rheotactic responses contribute to divergent habitat use between parapatric lake and stream threespine stickleback.

Migration among populations is widely thought to undermine adaptive divergence, assuming gene flow arises from random movement of individuals. If individuals instead differ in dispersal behavior, phenotype-dependent dispersal can reduce the effective rate of gene flow or even facilitate divergence. For example, parapatric populations of lake and stream stickleback tend to actively avoid dispersing into the adjoining habitat. However, the behavioral basis of this nonrandom dispersal was previously unknown. Here, we show that lake and stream stickleback exhibit divergent rheotactic responses (behavioral response to currents). During the breeding season, wild-caught inlet stream stickleback were better than lake fish at maintaining position in currents, faced upstream more, and spent more time in low-current areas. As a result, stream fish expended significantly less energy in currents than did lake fish. These divergent rheotactic responses likely contribute to divergent habitat use by lake and stream stickleback. Although rheotactic differences were absent in nonbreeding fish, divergent behavior of breeding-season fish may suffice for assortative mating by breeding location. The resulting reproductive isolation between lake and stream fish may explain the fine-scale evolutionary differentiation in parapatric stickleback populations.