“HKH screening”: a field bio-assessment to evaluate the ecological status of streams in the Hindu Kush-Himalayan region

Predation of fish assemblages in seagrass meadows was examined in the field and in tank experiments. Lure trolling indicated that (1) total abundance of fish was higher on bare sediment where small fish (<5 cm), including juveniles, predominated; (2) abundance was lowest in seagrass where large fish (>15 cm) predominated; (3) large ambush predators, primarily the grass goby and European eel, were almost completely restricted to seagrass; (4) the predation mode in seagrass was almost entirely ambushing or stalk-attacking, while the predation mode on bare sediment was almost entirely chase-attacking; (5) ambush predation was far more successful than chase-attack predation; and (6) overall predation risk was approximately three times higher in seagrass. Tank experiments showed that piscivory success of the grass goby was higher than that of the most common chase-attacker, the black goby, and the presence or absence of artificial seagrass, regardless of density, had no significant effect on predation success of either species. Guts of the grass goby contained food items of a wider size range that averaged twice the size of those of the black goby. Our results confirm our prediction that the risk of predation, especially of small/juvenile fish, is higher in seagrass meadows than at adjacent bare substrate, and this risk differential is explained by the presence of larger, more efficient ambush predators restricted to seagrass, and the scarcity of large chase-attack predators in the Novigrad Sea.