Sex Ratio and Maturation Characteristics of Adult Pacific Lamprey at Willamette Falls, Oregon
Returns of anadromous Pacific Lamprey Entosphenus tridentatus to the Columbia River have declined dramatically, with hydrosystem development and passage barriers identified as major factors contributing to these declines. Concurrent with interagency efforts to address issues associated with hydrosystem passage, the Columbia River treaty tribes have initiated adult translocation programs to supplement spawner numbers in interior streams, to rebuild natural production in the basin. The translocation programs involve collection of adults within fish ladders of lower mainstem dams, transportation of the fish to tribal facilities for holding overwinter (during which time the fish undergo final sexual maturation), and release of the fish the following spring into rivers and tributaries within their ceded territories where the lamprey were previously abundant but are now diminished in abundance or extirpated. Previous research on in-migrating Pacific Lamprey in the Willamette river, however, indicated that sex-ratios and maturation status of the fish may vary over the migration season. Inclusion of fish with a significant skew in sex ratio (especially if towards males) or holding of fish for overwintering that are already mature, could compromise the boost to natural production that is anticipated from the translocation programs. To obtain greater clarification of any trends in sex ratio and maturation status of migrating adult Pacific Lamprey, in 2016 we sampled 269 returning fish at Willamette Falls, Oregon City OR, that were collected as part of the annual tribal harvest. The fish were collected on one day per month in June, July and August – the same months during which adults are captured at the Columbia mainstem dams for the tribal translocation projects. Length, weight and dorsal fin gap measures were recorded for each fish, then they were dissected to identify sex, to record liver and gonad weight, and to make a visual assessment of gonadal development. Results indicated a sex ratio of 1:1 in all three samples (49F:51M; 34F:35M; 50F:50M, respectively). Gonadosomatic index (GSI) among males averaged 2.4% (range 0.4% to 5.7%) and no fish showed advanced testicular development. Among females, GSI averaged 7.2% (range 1.0% to 20.1%). A trend for decreased size and dorsal fin gap over time was observed for both sexes, as expected for fish initiating maturation following reentry into freshwater. Nonetheless, even the lamprey with the highest GSI values were not yet mature and would require at least one overwintering period before being ready to spawn. We also took the opportunity to collect tissue samples from these fish for eventual use in a study to identify a sex-linked DNA marker, and the heads from a random sub-sample were frozen, pending eventual extraction of the statoliths and examination to determine ocean age.
Porter, L.L., P.F. Galbreath, B.J. McIlraith, and J.E. Hess. 2017. Sex ratio and maturation characteristics of adult Pacific Lamprey at Willamette Falls, Oregon. Columbia River Inter-Tribal Fish Commission Technical Report 17-01. Portland, OR. 38p.