Host-Size Selection by Parasitic Sea Lampreys

Abstract

When 171 sea lampreys Petromyzon marinus were each offered a onetime choice of one large and two small lake trout Salvelinus namaycush, the ratio of their attacks (49.1%:27.5%: 23.4%) was similar to the ratio of host surface areas (51.4%:26.3%:22.3%) in laboratory tests conducted over 9 months in 1988–1989. When the smallest offered host was 560 mm total length (TL) or shorter, sea lampreys attacked the smallest host less often and the largest host more often than expected (N = 39) under assumptions that attacks would be either equal among the hosts (numbers-dependent hypothesis) or proportional to the hostsˈ surface areas (area-dependent hypothesis). When the smallest host was 615 mm TL or longer, observed attacks (N = 132) did not differ significantly from the number of attacks expected by either the numbers-dependent or the area-dependent hypothesis; however, the largest lake trout was almost always attacked more often than either of the smaller fish. These results are similar to field observations that sea lampreys kill few lake trout smaller than about 600 mm TL when larger fish are available as hosts, and that they attack larger fish (673–775+ mm TL) at a progressively greater rate as host size increases. Sea lamprey host-size selection was less pronounced in this study than in previous laboratory studies, which used smaller host fish than those now available in the Great Lakes.