Consequences of a superabundance of larval walleye pollock Theragra chalcogramma in the Gulf of Alaska in 1981

Abstract

Abundances of larval walleye pollock in Shelikof Strait, Gulf of Alaska, in 1981 were far greater than any recorded estimates before that time or since (some patch estimates exceeded 100000 larvae per 10 m 2 ). In spite of this extraordinary input, the ensuing 1981 year class was relatively poor. An examination of the feeding habits of larvae collected from inside and outside dense larval patches revealed that in 1981, larval walleye pollock consumed significantly more invertebrate eggs (x = 66.7% of the total diet) and fewer copepod nauplii (x = 13.6%) inside of larval patches relative to outside (x = 11.4 and 63.2%, respectively). These observations suggest that density dependent com- petition inside patches may have locally depleted the primary food source, copepod nauplii, prompting a diet switch to a lower quality but more abundant prey resource, invertebrate eggs. Results from a bioenergetics simulation support this theory, indicating that dense patches of walleye pollock larvae in 1981 were capable of exhausting naupliiar prey resources in Shelikof Strait in a relatively short period of time (14 to 40 d). Our observations contrast with other studies that suggest that ichthyoplankton exert little or no influence on microzooplankton standing stocks. Rather, we present evidence that, in certain unusual circumstances, particularly dense aggregations of larvae are capable of locally de- pleting the prey community. Such events may weaken larval condition and exacerbate natural death rates, which may have contributed to the poor recruitment success of the 1981 year class.