Neomysis integer in a shallow hypertrophic brackish lake: distribution andpredation by three-spined stickleback (Gasterosteus aculeatus)

The mysid shrimp Neomysis integer is a common invertebrate predator in brac kish waters of Western Europe and is thought to play a central role in the food web owing to its predation on zooplankton. Neomysis distribution and a bundance were investigated for 3 years in brackish, shallow and hypertrophi c Lake Ferring (surface area: 3.2 km(2), mean depth: 1.4 m, salinity: 3-6 p arts per thousand, total P: 0.29-0.78 mg P l(-1), Secchi depth: 0.14-0.22 m ). Mean summer abundance of Neomysis varied from 53 to 882 ind. m(-2). Neom ysis density within the lake was relatively uniform and not related to sedi ment type, but increased with increasing depth. The high abundance of Neomy sis is considered to reflect a fish stock almost completely dominated by sm all-sized fish species (mainly three-spined stickleback, Gasterosteus acule atus). Three-spined stickleback density was high and catch per unit effort ranged between 30 and 80 per gill net. Stomach analyses showed that the sti cklebacks preyed on Neomysis, but preferred specimens smaller than 3-4 mm, and only occasionally consumed those larger than 5-7 mm. In summer, between 33 and 67% of the Neomysis ingested by sticklebacks were smaller than 3 mm , while in the lake as a whole, only 5-14% were smaller than 3 mm. The peri ods when Neomysis is vulnerable to stickleback predation are restricted to a few weeks in late May and late July, when the new generations emerge. Sti cklebacks therefore have a limited capacity for controlling large Neomysis, including gravid females.