Larval fishes in relation to water masses of the Central North Pacific transitional areas, including the shelf break of West-Central Alaska

Larval fishes and corresponding physical data were sampled along two transects from Seward, Alaska to Oahu, Hawaii in February 1991. The first transect was west along the shelf break (SH) off Kodiak Island, 57 degree 38' to 55 degree 20' N, between 149 and 156 degree W. The second transect, North Hawaii (NH), was south along 158 degree W from 54 degree 40' N, at Shumigan Islands off the Alaskan Peninsula, to 22 degree 00' N, near the island of Oahu, Hawaii. No other cruise has specifically sampled fish larvae along a similar transect from Alaska to Hawaii in the winter. Myctophidae, Gonostomatidae and Sternoptychidae dominated our catches. A variety of methods have been used to assign faunal provinces and to define distribution of fishes, from descriptive to empirical to statistical to physical. The result is that zoogeographic zones in the North Pacific have been defined in several ways, depending on the taxa and area studied. The multiplicity of nomenclature complicates comparisons, however the difference may be a matter of semantics. We used Roden's (1991) salinity and temperature limits as an objective method for determining water mass boundaries and analyzing faunal provinces. The subtropical domain had the highest diversity, as expected; however, the subarctic front was a close second. Diversity was the lowest in the subarctic domain and the transition zone. The counter-intuitive low values of diversity and evenness in the transition zone are explained by the vertical distributions of fish in these water masses. When temperature divisions are used to calculate diversity, a true "transition zone" appears between 8 and 19 degree C. The "transition" of taxa apparently occurred north of the physically derived transition zone. This may be a situation unique to the relatively wide transition zone of February 1991. If not, then future analysis should include frontal zones as part of one large transition zone to reflect the broader subarctic-subtropical transition zone. Regardless of the label or the method used to assign faunal provinces, the distribution of larval fishes is regulated by water mass characteristics.