RESPONSE OF A MOBILE INVERTEBRATE 519 



Another feature of H. azteca's distributional response to temper- 

 ature was the fall migration of amphipods into previously unavailable 

 habitats. Within two months, amphipods were found in samplers 

 3 km away from their original site of dispersal. We observed that 

 amphipods colonized samplers sequentially from the cool to the 

 warm end of the lake; this observation suggests that migration is 

 influenced by temperature. Kromrey et al. (1976) reached similar 

 conclusions in a study of weekly changes in the distribution and 

 abundance of H. azteca in Lake Columbia. Although H. azteca are 

 known to make diel migrations from the littoral zone to areas 

 offshore (Mullen, 1969), extensive migrations similar to those in 

 Lake Columbia have not been reported in the literature. Laboratory 

 experiments and field data gave corroborating evidence of size- 

 dependent activity. 



Size-selective predation by fish must be considered in inter- 

 preting data comparing amphipod sizes with their locations in the 

 lake (Strong, 1973). Larger amphipods are presumably preferred 

 prey. However, concomitant studies of fish distribution and 

 abundance revealed that fish densities are greatest at stations where 

 temperatures are higher (> 28° C) than those tolerated by amphipods 

 (Lozano, Rondorf, and Kitchell, 1978). If predation were important, 

 we would expect an inverse relationship between predator density 

 and prey size. In fact, the reverse is true. The largest amphipods were 

 proportionately more abundant in samples taken from stations where 

 fish were most abundant. We are unable to determine whether 

 selective predation on small amphipods was occurring, but the more 

 probable explanation is simply that vagility is size dependent. Larger 

 amphipods are probably more mobile and, hence, represent a greater 

 proportion of the population of a newly colonized habitat (Table 1). 



Seasonal changes in life-history data for amphipods collected in 

 the settling basin (Fig. 3) were similar to those described by other 

 investigators (Cooper, 1965; Mathias, 1971; Strong, 1973). Repro- 

 duction of amphipods in the settling basin began in late May or early 

 June and continued through September. Population abundance 

 increased very rapidly in June and then declined throughout the 

 summer. High summer mortality is common for H. azteca (Cooper, 

 1965). There is also evidence of seasonal bimodality in abundance 

 during the summer and fall. The high abundance of amphipods from 

 September through November could have resulted from lower 

 mortality rates during the cool months for amphipods born in late 

 August and September. 



Amphipod life-history features in the cooling lake differ from the 

 typical annual pattern seen in aquatic invertebrate populations with 



