Substrate-related differences in anti-predator behaviour of two gobiid fish species and the brown shrimp, and their adaptive value

Abstract

Factors affecting the predator-related mortality of sand goby Pomatoschistus minutus (Pallas), common goby P. rnicrops (Kreyer) and brown shrimp Crangon crangon L. were investigated in Gullmar Fjord, Sweden. Substrate selection and locomotory activity was recorded in the 3 species on sand and on mud. Sand goby and brown shrimp preferred sand to mud, while common goby preferred mud to sand. Activity was clearly correlated to substrate type. In all 3 species, swimming activity was increased by a factor 4 when kept on the less preferred substrate. Predation experiments were performed with sand or mud as bottom substrate and with cod as predator. On sand, common gobies were more rapidly eliminated by the predator than were sand gobies, and sand gobies more rapidly than brown shrimps. On mud, sand gobies were more rapidly taken than were common gobies and brown shrimps. Staying on the 'wrong' substrate will result in increased locomotory activity which will in turn increase the risk of predation. We suggest that the microgeographical distribution of members of the epibenthic guild is primarily a consequence of various behaviour patterns. Predators act to restrict the habitat use of these fish and shrimp species. Differences in behaviour between the species when encountering a predator reduce their vulnerability to predators on their respective preferred substrate. This may in part explain the spatial segregation and will contribute to the maintainance of community structure. MARINE ECOLOGY PROGRESS SERIES Mar. Ecol. Prog. Ser. INTRODUCTION Published March 26 Selective predation by fish has been demonstrated to influence community structure in marine and freshwater ecosystems (e.g. Brooks & Dodson 1965, Wells 1970, Dodson 1974, Nelson 1979, Stoner 1979). Predators may control microdistribution of prey not only through selective elimination but also through intimidation (Charnov et al. 1976). Many studies have dealt with the foraging strategies of the predator (for reviews see Curio 1976, Macan 1977), but considerably fewer have specifically dealt with the antipredator behaviour of the prey. In the marine, shallow soft-bottom environment, the presence of vegetation increases habitat complexity and reduces predator-related mortality for small fish and crustaceans (Nelson 1979, Stoner 1979. Berglund & Bengtsson 1981, Coen et al. 1981, Heck & Thoman 1981. Mine110 & Zimmerman 1983). On bare sand, on the other hand, shelter is lacking and the risk of predation might be one of the many factors that limit the habitat use of epibenthic species. Various antipredator O Inter-Research/Printed in F. R. Germany behaviours developed by potential prey species are of great survival value and contribute to the segregation of species occupying different substrates. On the Swedish west coast the dominant species of the epibenthic guild on unvegetated bottoms are the sand goby Pomatoschistus minutus (Pallas), the common goby P, microps (Krayer) and the brown shrimp Crangon crangon L. (Evans & Tallmark 1979, 1984, 1985). Generally, sand goby and brown shrimp are dominant on sandy bottoms whereas common goby is most abundant on muddy bottoms. All 3 species show a wide overlap in dietary habits and only a small temporal segregation in peak abundances. This will make it possible for them to exploit resources available when one of them temporarily is reduced in number or even absent (Evans & Tallmark 1985). Our previous results indicated that interference competition, where space is the proximate object of competition, is important in maintaining both structure and size of the epibenthic guild. In this paper we evaluate the role of behaviour and predation in small-sized fish and shrimp species on shallow bottoms, and concentrate on the following 218 Mar. Ecol. Prog. Ser. 29: 217-222, 1986 questions: (1) Does the substrate affect the predation rate of these species? (2) Are there any differences in antipredator behaviour between these species, and if so, are they substrate dependent? (3) Is there a correlation between the antipredator behaviour of these small-sized fish and shrimp species and the predation