OBSERVED PATCHES OF WALLEYE POLLOCK EGGS AND LARVAE IN SHELIKOF-STRAIT, ALASKA - THEIR CHARACTERISTICS, FORMATION AND PERSISTENCE

Using observations from 38 ichthyoplankton surveys conducted near Shel ikof Strait, Alaska between 1979 and 1992, we characterized the horizo ntal distribution and spatial patchiness of the early life stages of w alleye pollock (Theragra chalcogramma). Lloyd's index of patchiness ra nged from 3.9-6.1 for eggs and 3.9-16.2 for larvae. This index was siz e (age) dependent: low for eggs, high for newly hatched larvae, then d ecreasing through late larval stage. By the early juvenile stage, patc hiness increased as pollock began to school. The percentage of larvae in a patch (defined as the percentage of larvae present at stations wh ere larval counts exceeded the mean by one standard deviation during t he given survey) varied greatly (26-92%). Larval distributions were us ed to deduce physical mechanisms responsible for patches. Three catego ries of patches were identified: those created by interaction of larva e with time-dependent currents, those in the vicinity of Sutwik Island , and chose associated with eddies. Simulation experiments were utiliz ed to examine processes influencing patch formation and the role of la rval swimming. Between 5 and 6 weeks after hatching, larvae have swimm ing abilities that enable them to maintain a patch already created by physical mechanisms.