Male–male competition and female mate choice may both play important roles in driving and maintaining reproductive isolation between species. When previously allopatric species come into secondary contact with each other due to introductions, they provide an opportunity to evaluate the identity and strength of reproductive isolating mechanisms. If reproductive isolation is not maintained, hybridization may occur. We examined how reproductive isolating mechanisms mediate hybridization between endemic populations of the Red River pupfish Cyprinodon rubrofluviatilis and the recently introduced sheepshead minnow C. variegatus. In lab-based dominance trials, males of both species won the same number of competitions. However, male C. rubrofluviatilis that won competitions were more aggressive than C. variegatus winners, and more aggression was needed to win against competitor C. variagatus than allopatric C. rubrofluviatilis. Duration of fights also differed based on the relatedness of the competitor. In dichotomous mate choice trials, there were no conspecific or heterospecific preferences expressed by females of either species. Our findings that male–male aggression differs between closely and distantly related groups, but female choice does not suggest that male–male competition may be the more likely mechanism to impede gene flow in this system.

The Channel Catfish Ictalurus punctatus is a popular sport fish that is routinely stocked into north-temperate U.S. lakes and reservoirs by natural resource management agencies. We experimentally tested the use of an internal coded wire tag (CWT) as a method for nonlethal batch marking of individuals. The retention of CWTs and their influence on individual growth and mortality remain unexplored for juvenile Channel Catfish. Both attributes were quantified and compared between age-0 fingerlings and age-1 yearlings at various time points (i.e., after 1 week, 2 weeks, 1 month, and monthly thereafter) of our 4-month experiment. For both age-classes, CWT retention was high (90-100%), with no observed increases in mortality or effects on growth rate. Our findings suggest that CWTs are a reliable, effective means of marking juvenile Channel Catfish. Hence, we recommend their use by management agencies to differentiate between hatchery-reared and wild-produced individuals, which can be useful when assessing the efficacy of stocking programs.

Management agencies commonly stock Channel Catfish Ictalurus punctatus as advanced yearlings instead of fingerlings (age 0) to help reduce potential predation mortality despite uncertainty regarding the degree to which predation limits fingerling poststocking survival. To better understand whether fingerling survival during the first 7 d after stocking—when predation risk is likely greatest—could benefit from the presence of alternative prey fish, we conducted an experiment in 0.4‐ha ponds (N = 18; 6 ponds/treatment) that measured fingerling (TL range = 56–186 mm) survival in ponds with predators only (adult Largemouth Bass Micropterus salmoides; mean TL = 327 mm; SE = 21), with predators and alternative prey fishes (Bluegill Lepomis macrochirus and Goldfish Carassius auratus), or with neither (i.e., control ponds). Without alternative prey, fingerling Channel Catfish survival 1 week after stocking was significantly (P < 0.01) lower (89%) than in the ponds with alternative prey (98%) or in the control treatment with no predators (97%). Our experiment also indicated the potential for alternative prey to promote fingerling survival by reducing water clarity, not just by serving as a buffer from predation. Further research, however, is needed to determine the degree to which our findings apply to larger ecosystems and to learn whether stocked fingerlings survive well enough after the initial stocking period to achieve desired management goals. Even so, our results suggest that alternative prey can reduce Largemouth Bass predation mortality on fingerlings immediately after stocking, which could influence Channel Catfish stocking approaches in some managed systems.