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~ • } American Fisheries Society Symposium 5:96-103, 1988 © Copyright by the American Fisheries Society 1988 Abundance, Distribution, Growth, and Mortality of Larval Lake Herring in Western Lake Superiorl JAY T. HATCH General College, University of Minnesota, Minneapolis, Minnesota 55455, USA JAMES C. UNDERHILL Department of Ecology and Behavioral Biology, University of Minnesota Minneapolis, Minnesota 55455, USA Abstract.-Abundance, distribution, growth and mortality of larval lake hemng Coregonus artedii were studied in the Duluth (Minnesota) to Superior (Wisconsin) area of western Lake Superior. Larvae were present in the area from late April to mid-June but were most abundant from mid- to late May. Normally, larvae were much more abundant in the upper 2 m of water than in the 4-6 m stratum. Mean densities of larvae in the vicinity of the Duluth harbor entry were 10-100 times greater than those in other areas and often exceeded 1,000/1,000 m' of water. Larvae remained in nearshore areas for 14-20 d after hatching; during this time they grew from about 10 mm standard length to 20 mm and reached the late mesolarval phase of development. Preliminary estimates of instantaneous growth rates for protolarval through mesolarval phases were 0.054- 0.055, and total instantaneous mortality rates were 0.41-0.59. The Duluth-Superior area is an important developmental area for larval lake herring and offers excellent potential for developing larval abundance index stations. Lake herring Coregonus artedii populations in Lake Superior have declined precipitously since the mid-1940s (Baldwin et al. 1979). Selgeby (1982) presented a convincing argument that, in the case of western Lake Superior populations, the decline occurred as a direct result of overex- ploitation. However, despite a great reduction in fishing intensity for these populations from the late 1960s to the present, lake hemng abundance has remained relatively low. The reasons for the failure of western populations to recover are not clear. Anderson and Smith (1971) concluded that competition for zooplankton between the larvae of lake herring and rainbow smelt Osmerus mor- dax and, to a lesser extent, bloater Coregonus hoyi had a strong negative ,influence on lake herring abundance. More recently, Swenson and Heist (1981) suggested that rainbow smelt preda- tion on larval lake herring could account for the lake herring's continued low abundance in the Duluth-Superior area. Selgeby et al. (1978) found that rainbow smelt predation was imporant in Black Bay, Ontario, but not in the Apostle Islands (Wisconsin) region. These studies suggest the hypothesis that high mortality during the larval period may be one reason that recruitment of adult lake herring has remained low. At the very 'Research Contribution 203 of Minnesota Sea Grant, University of Minnesota, St. Paul. least, these studies and others (Faber 1970; Lind- strom 1970; Reckahn 1970; Viljanen 1980) clearly point to the need for a greater understanding of the ecology of larval coregonines and of how their growth and mortality affects adult population size. The present study was undertaken as a first step in elucidating the dynamics of larval lake herring populations in the Duluth-Superior area of west- ern Lake Superior. Our immediate objective was to investigate the spatiotemporal distributions of lake hemng larvae in the area- so that index stations for measuring larval density, growth rates, and mortality rates could be established. We did not intend to estimate growth and mortal- ity rates from this pilot study. ~ ~:Yc~er, since the project was not continued, we made preliminary estimates to determine if catch-curve analysis could be used for this purpose. We present data on the seasonal occurrence and distribution of lake herring in the study area, along with esti- mates of larval density in six index zones and the first field estimates of instantaneous growth and total mortality rates for larval lake herring. Methods Field sampling.-Sampling was conducted dur- ing May and June 1981 and April-June 1982 at various stations in the Duluth-Superior area of western Lake Superior (Figure 1). Sampling in 1981 was primarily exploratory; therefore, the 96