ABSTRACT
Generally, gobies are carnivorous, oft en algae or other plants are ingested as secondary, rarely as primary prey. In some cases, these fi sh are aufwuchs feeders, which ingest plants as well as sessile animals like polychaetes or sponges, e.g. Gobius vittatus from the Gulf of Lion (Heymer and Zander, 1978). Th e kind of prey especially depends on two factors of the predator: size and way of life. Large species feed rather on large animals including fi sh, small species on small organisms. Th is rule can at best be demonstrated by ontogeny. Large gobies of more than 20 cm size are represented, e.g., by the Mediterranean Gobius cobitis, Zosterisessor ophiocephalus, the Pontocaspian genera Mesogobius and Neogobius, or by species of the family Eleotridae. But the most gobies attain only sizes of 8-12 cm. Beside species which live several years, there are many gobies which spawn in the year aft er hatching and die thereaft er; these species grow very fast to a size of 6 cm, some even to 9-10 cm (Pomatoschistus minutus), some to only 2 cm or less (Pandaka pygmaea). Secondly, the respective way of life defi nes also
the layer of foraging in the benthal, suprabenthal or pelagial. Whereas the largest gobies are bottom dwelling species, we fi nd several smaller species which live in the suprabenthic (e.g. Gobiusculus fl avescens, Aphia minuta, Pomatoschistus quagga), sometimes also in the pelagic layer (Crystallogobius linearis) and, therefore, feed mainly on planktonic organisms. Additionally, the suprabenthal is also exploited by benthic species, e.g. by Pomatoschistus pictus which regularly starts from the bottom into swarms of mysids in order to catch single specimen of these crustaceans. Expressed as an ecological formula, the demand and acquisition of prey follows the energetic rule of MacArthur and Pianka (1966): Eopt = Ei / Ts + Th , where Eopt = optimal energy acquisition, Ei = energy content of prey, Ts = searching time, Th = handling time which includes persecution, overwhelming and feeding. An optimal concept, whch is also used by large gobies, is that of sit and wait for relatively large prey, because the predator gains much energy but invests a minimum of handling time. In contrast, plankton feeders, so called widely foragers, gain only low energy by a single tiny prey, but this is compensated by a minimum of searching time. Regularly, the ontogeny of gobies begins aft er hatching with a pelagic phase during which the goby larvae feed on planktonic organisms. Th e duration of the pelagic phase varies according to the ontogeny patterns of the respective species. During their growth, they forage corresponding to their size at fi rst for small invertebrate larvae and later on greater organisms, like adult Copepoda. Regarding taxonomic units, the food spectrum is wide wherein the preferred group is clearly the Crustacea resulting from their great species richness and abundance in the aquatic environments, especially in marine habitats. In freshwater these are partly replaced by water dwelling insects or their larvae. Th e prey demand can be completed by Polychaeta, Mollusca and fi sh from the bottom or larvae of diverse groups from the pelagial. Th e most important factors, which may infl uence and probably restrict the prey selection of these fi sh, are the quality and quantity of prey off er. In extreme habitats the potential food organisms can consist of only few species which occur in great abundance following Th ienemann’s (1939) biocenotic rule. Such environments like brackish waters, mud flats or mangrove swamps present high productivity and prey surplus. Gobies as generalists will then prefer those organisms which are due to their dominance caught easier and reduce searching time according to the energetic rule (MacArthur and Pianka, 1966). Th us, the predator seems to be a specialist dependant only to a dominant prey species. Other extreme
habitats like caves comprise a small but diverse prey off er, which make the impression of an extreme generalist. If new environments like land or fresh water are conquered these may serve as an expansion to a habitat which is poor of competitors. Special ways of life like cleaning hosts from parasites consequently reduces the food spectrum on ectoparasites, but does not reduce the possibility to feed on other food items. Th e productivity of an ecosystem can be mirrored by the fullness of the gut of a predator, expressed in the “fullness index” (Hureau, 1970): Full = Wf / Wp , where Wf = weight of food in the gut of a single predator, Wp = weight of the predator. Th e analysis of food spectra is focussed on three parameters which are set in relation: 1. frequency of occurrence (Hynes, 1950) which marks the importance of a food item for the predator population: F = Ni / N , where Ni = number of predators with a special food item / N = total number of predator specimens; 2. abundance which mark the numerical dominance of a food item: Ab = Si / N, where Si = number of a special food item; 3. weight which marks the aff ectivity and nutritive value of a food item: W = Wi / N , where Wi = weight of a special food item in a predator population. Several investigators use the volume “V” or volume of a food item “Vi” of prey as an alternative parameter to weight. Th e meaning of the respective parameters is diff erent. Th us, the frequency may mirror the abundance of the off ered food items, the abundance can express feeding activities (handling time), the weight or volume is the most important factor for the nutrition of a predator (Zander, 1982). Several authors tried to combine two or three compartments with the aim to improve these analyses and to develop equations in order to characterise the signifi cance of food items. In his nutrient coeffi cient, Hureau (1969) multiplied percentage of numbers with percentage of weight. Th e index of relative importance (Pinkas et al., 1971) uses still the volume instead of weight: IRI = % F (% Ab + % V). Simenstad (1979) replaced volume by weight. Because Zander (1982) assumes that frequency in the IRI is overvalued and volume is too inaccurate for small-sized fi sh, he developed the main food index: MFI = √ [(% Ab + % F) % W / 2], where W = weight which is emphasised to use in small-sized fi sh like gobies as dry weight. Th e calculated values are classifi ed as follows: > 75 = primary food, 5175 = main food, 26-50 = secondary food, < 25 = insignifi cant food. Th e advantage lay in the presentation of the MFI by 3-axis graphs see below. Because volumes in IRI are only coarsely estimated, Nieder and Zander (1994) present a method of a more accurate evaluation which is especially suited for small-sized fi sh. Finally, the calculation of selectivity can fi nd out the preferred food items of the predator by the following equation
(Berg, 1979): Sel = log (number of ingested food / number of potentially available food). Numbers can be replaced in more accurate studies by weight. Th e calculation of Sel needs samples of free living communities which are only rarely done. Because most species are of small size, gobies are a preferred prey for greater fish (also greater gobies), diving birds and mammals (seals and dolphins), but also for cuttlefi sh, jellyfi sh and anemones. Species which inhabit shallow waters are prey even for not diving birds like grebes and gulls. By this way, parasites can perform their life-cycles from intermediate (gobies) to fi nal hosts (predator fi sh, birds and sea mammals). Gobies are also exploited by human fi shery, which regards especially larger species in the Mediterranean and Black Sea, but also in tropic areas on tiny species like Pandaka pygmaea, which is one of the smallest vertebrate.
