Individuals were collected from a residential marine population of Spinachia spinachia, an anadromous population of Gasterosteus aculeatus forma trachura and a residential freshwater population of G. aculeatus forma leiura. After maintenance for 2 months on a diet of mysid, individuals were subjected to ten, consecutive daily trials on a diet of amphipods or oligochaetes. During this period, individuals learned to handle the prey more effectively, as measured by attack efficiency, handling efficiency and handling time. Learning was similar among populations but differed between diets, being more pronounced for amphipods, which are more difficult to catch and handle than oligochaetes. Once trained to these diets, fish were tested for foraging efficiency after successively longer periods of stimulus deprivation, when they were fed a maintenance diet of mysid. All three measures of foraging efficiency with the amphipod diet, but only that based on handling time with the oligochacte diet, declined to naive levels in the residential marine and anadromous populations. No decrease in foraging efficiency with either diet occurred in the residential freshwater population. Memory window was 8 d, 10 d and > 25 d in the residential marine, anadromous and residential freshwater populations respectively. The large difference between the freshwater and two marine populations is interpreted as an adaptive response to the stability of arrays of prey, characteristic of their respective habitats.