Food Resource Partitioning Between Sympatric Populations of Brackishwater Sticklebacks

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Abstract
(1) The diets of juvenile and adult sympatric sticklebacks (Apeltes quadracus, Gasterosteus aculeatus, G. wheatlandi and Pungitius pungitius) were compared with the abundance of potential prey in the environment. (2) The sites exhibited differences in salinity and temperature and these were related to the observed differences in stickleback population structure. (3) Site A had a greater abundance of potential prey than site B. Within each site, areas with abundant aquatic vegetation had higher densities of potential prey than vegetation-poor areas. (4) Adults of all four species appeared in the breeding areas in early May and were no longer present by the end of June. Juveniles appeared in late June, except for juvenile G. wheatlandi which did not appear until July because of their later breeding period. Juvenile Gasterosteus left the breeding areas by late August, while juvenile A. quadracus and P. pungitius remained. (5) Food niche overlap was greatest between adult G. aculeatus and P. pungitius, while adult G. wheatlandi overlapped least with any species. Where A.quadracus was found in sympatry with the other species, overlap occurred with G. aculeatus and P. pungitius. The same relationships held for juveniles. (6) In four of the five areas, niche breadth and overlap were greater in areas of high potential-prey density and lower in areas of low potential-prey density. In the fifth area, niche overlap was high at low potential-prey densities. Because niche breadth varied between sites, no firm conclusions could be drawn about niche breadth relationships between the species. (7) Adult and juvenile sticklebacks exhibited similar microhabitat partitioning, although it was more pronounced in juveniles. A. quadracus fed predominantly from the benthos, while G. wheatlandi fed mainly in the water column. G. aculeatus and P. pungitius fed from both the benthos and water column, P. pungitius usually foraged amongst algal mats. (8) It is proposed that interspecific competition was not a factor in these environments, because of the abundance of potential prey and microhabitat segregation.