PS 50-166 - Seasonal variation in sport fish abundances at three reservoirs in central Arizona

Background/Question/Methods

Seasonal variation across different ecosystems is a fairly normal occurrence in the environment and can cause dramatic shifts in species abundances. Changes in sunlight, temperature, precipitation, and other variables cause shifts in the environment by decreasing or increasing the resources available and therefore impacting the organisms in the ecosystem. In Arizona, the desert climate does not fluctuate as drastically as in other regions, however, temperatures do get much hotter in the summer and winters are mildly cool. The objective of our study was to examine seasonal changes in fish populations across three Arizona reservoirs (Lakes Pleasant, Roosevelt, and Apache). We used pelagic gill nets to sample fish populations in these reservoirs across three seasons (Fall, Spring, Summer) and focused our analyses on the most common and economically-important sport fish (Channel Catfish, Yellow Bass, White Bass, Largemouth Bass). We compared the average total abundances of each of these fish species for each lake and each season. In addition, we were able to compare the total sport fish abundances (across all species encountered in gill nets) as well, to determine if sport fish generally varied by season in Arizona reservoirs.

Results/Conclusions

Overall, we did not see any seasonal variation in fish abundances in either Apache Lake (F = 0.018, p >0.05) or Lake Roosevelt (F = 0.701, p >0.05). However, we did see differences in sport fish abundances by season at Lake Pleasant (F = 4.502, p< 0.05). Looking closely at the most abundant species in Lake Pleasant, we saw that White Bass was the only species that varied significantly by season (F = 7.026, p< 0.05). White Bass is known to vary in catch rates over time because it is a temperate species that travels upstream during spring spawning. Its population in Lake Pleasant decreased in the spring and summer, and increased in the fall, likely as a result of spawning migrations out of the pelagic part of the lake. Generally, we found little predictable variation in fish abundances by season, despite pronounced seasonal variation being documented in numerous other studies of reservoir sport fishes. Part of this pattern may be because most Arizona reservoirs do not freeze over in winter like in other places. More research is needed to look at seasonal variation across a broader climatic range in order to understand how it affects fish populations across seasons.