Multispecies larval fish associations: Accident or Adaptation?

Abstract

Capelin larvae (Mallotus villosus) time their period of larval drift to coincide with water masses containing abundant food resources and a reduced predator complex by responding to environmental cues associated with coastal water mass replacement. This persistent, coast-wide phenomenon led us to hypothesize that the initiation of larval drift in species of fishes other than capelin, having similar spawning modes, should be linked to the occurrence of these "safe site" water masses. The abundance of larvae of demersal spawning species, other than capelin, in the nearshore waters of our study site in eastern Newfoundland was highly variable during June–August in 1979 and 1980 and was unrelated to seasonal trends. The abundance of this sympatric noncapelin component of the ichthyoplankton was, however, positively related to both water temperature and larval capelin density, both of which are strongly influenced by coastal water mass replacement, and to the occurrence of conditions favorable to larval survival: "safe sites." The noncapelin larval assemblage was dominated by species having demersal eggs (90% of the total catch in two years) spawned in shallow water, in particular, winter flounder (Pseudopleuronectes americanus), radiated shanny (Ulvaria subbifurcata), and seasnails (Liparis spp.). Weekly size frequency distributions of these three species were dominated by newly hatched larvae. We conclude that the onset of larval drift in these species was also synchronized to the temporally variable occurrence of ecological safe sites for larvae. Three advantages accrue: food levels and potential growth rates would be greatly enhanced, predator density would be reduced, and predation rates would be reduced due to association with the dominant species (capelin) that saturate the predator field. Similar associations between a single dominant species and other much less abundant species appear to be common features of the ichthyoplankton of temperate/boreal regions.