other cases of benefit or of neutral effect to human activity. In the disruptive category fall incidences of dolphins taking fish and invertebrates out of nets and long-lines, and into the beneficial category fall incidences of humans and dolphins apparently cooperating in order to herd or drive prey in some manner, usually against a shoreline. A particularly well-described account has been given of killer whales helping whalers of New South Wales ambush and drive humpback whales ( Dakin, 1934). Nevertheless, in these accounts it is usually not possible to separate whether or not the actions of the two species were designed to be cooperative, or whether they were in their own ways simply working towards the common goal of herding or trapping the same prey. The fishing people involved in these interactions are usually very certain that they are cooperating with the dolphins, and they also believe that the dolphins are not simply taking blatant advantage of the human activity, but are exercising "temporary restraint" ( Wilson, 1975) in order to herd prey and postpone feeding in a manner equitable to both groups. Neutral interactions involve dolphins feeding off human garbage ( Norris & Prescott, 1961) or following shrimp boats while they are hauling in or sorting their catch ( Gruber, 1981; Fulton, 1976, provides a particularly clear account of dolphins around a shrimp boat).
This brief and incomplete overview of delphinid foraging strategies tells us that there is quite a bit of diversity between species, and for at least some species such as the bottlenose dolphin, quite a bit of feeding diversity within the same animal. The situation is not unlike that of social mammals such as wolves, wild dogs, and lions on land. These animals often cooperate in securing prey, and they do so not always in stereotyped fashion ( Wilson, 1975, provides several good reviews; also Kruuk, 1972; Schaller, 1972). Our observations of foraging strategy related to the immediate finding and securing of prey tell us nothing about the possibility of such overall strategies as, for example, optimization of energy supplies by pulse fishing ( Norris & Dohl, 1980a).
Cooperative foraging is not, of course, a trait only of social mammals. Excellent recent observations have shown that some predatory fish are more successful at catching schooling prey when they themselves are schooling ( Major, 1976, 1978). Schmitt and Strand ( 1982) recently discussed cooperative foraging by the marine piscivorous yellowtail (Serriola lalandei). Yellowtails surround and herd jack mackerel (Trachurus symmetricus) toward shore, and they herd Cortez grunts (Lythrulon flaviguttatum) into open water. These two fish species are open-water fast swimming and nearshore reef-hiding species, respectively, and it appears that yellowtails change their strategies in order to herd fish to areas where their normal defenses of speed or hiding can no longer be used. The yellowtails show true cooperation, as in much cooperation in foraging mammals,