Behav Ecol Sociobiol(1990) 26:331-335 Behavioral Ecology and Sociobiology © Springer-Verlag1990 Reproduction under predation risk in the sand goby, Pomatoschistus minutus, and the black goby, Gobius niger: the effect of age and longevity Carin Magnhagen Department of Zoology, Uppsala University, Box 561, S-751 22 Uppsala, Sweden ReceivedAugust 17, 1989 / Accepted December 13, 1989 Summary. In aquarium experiments, the two marine go- biid fish species Pomatosehistus minutus and Gobius niger were allowed to build nests and to spawn in the presence and absence of a predator (cod, Gadus morhua); behind a glass wall the predator was kept where it could be clearly seen by the gobies and vice versa. P. minutus showed no difference in number of nests or number of spawnings in the different treatments; approximately half of the males built nests, and the females spawned in half of those nests. G. niger, on the other hand, re- sponded differently to the simulated predation risk. No nests were built in sight of the predator, whereas in the absence of predators, half of the males built nests and received eggs. The G. niger individuals in this experiment were 2-3 years old. However, when comparing the re- production of G. niger of different age in the presence of a predator, older individuals (4-5 years) spawned, whereas younger (2-3 years) did not. No difference in vulnerability towards predators was found between equal-sized P. minutus and G. niger. The optimal behav- ior during the breeding season must depend on prospects of survival, based both on maximal lifespan and vulnera- bility to predation. Introduction Animals have several factors to consider in order to opti- mize their behavior. There are often conflicts between different demands, such as foraging, reproduction, and predator avoidance (Sih 1980; Cerri and Fraser 1983; Dill and Fraser 1984; Ryan 1985; Endler 1987). For example, foraging rate and habitat choice are commonly found to be influenced by predation risk (Milinski and Heller 1978; Krebs 1980; Dill 1983; Lima et al. 1985; Metcalfe et al. 1987). Foraging behavior can increase the animal's susceptibility to predators. Also during repro- duction an animal may be more vulnerable to predation than otherwise, either because of a higher visibility caused by egg-carrying (Winfield and Townsend 1983; Berglund and Rosenqvist/986; Svensson t988) or breed- ing coloration (Wootton 1984; Endler/982), or due to conspicuous displays that attract predators (Ryan et al. 1982; Burk 1982; Giles 1984; Helfman 1986). However, little is known about how reproductive behavior is in- flueffced by predation risk. According to dynamic foraging theories (Mangel and Clark /986; McNamara and Houston 1986) the state of the forager (i.e., energy reserves or hunger levels), correlated with the probability of survival, determines the amount of risk that is accepted; these theories are supported by empirical studies (Dill and Fraser 1984; Magnhagen 1988a, b). Similarly, when a trade-off has to be made between reproduction and predator-avoid- ance, the degree of risk-taking during the present repro- ductive event should be related to the probability of future reproduction (Pugesek 1981; Tallamy 1982). One of the most important components of life-time reproduc- tive success across most animal taxa seems to be longevi- ty (Clutton-Brock 1988). The optimal behavior should therefore differ between species with different longevity and within species between individuals of different age (Sih 1987), with shortlived species and older individuals taking higher risks in order to reproduce. In species of the fish family Gobiidae, males exhibit parental care. They build nests (using, e.g., mussel shells or rocks, depending on species), use display to attract females to their nests to spawn, and guard and fan the eggs until hatching (Miller 1984). Both nest building and courtship should make male gobies conspicuous and hence vulnerable to predation (Potts 1984), and one would assume that they should be able to assess current predation risk and to adjust their behavior accordingly. The sand goby, Pomatoschistus minutus, and the black goby, Gobius niger, both occur abundantly in shal-