Characterizing seasonal dormancy and the physiological threshold for survival in Pacific sand lance (Ammodytes personatus)

Abstract

Pacific sand lance (Ammodytes personatus) go dormant through the winter. As foraging decreases, sand lance experience a decline in body condition. Natural mortality may occur due to starvation if body condition decreases beneath a physiological “threshold of survival”. To quantify the threshold, juvenile Pacific sand lance were gathered from Jackson Beach on San Juan Island, Washington, and adults were collected from the San Juan Channel sand wave field. Fish were placed in tanks separated by life stage/source location. One juvenile tank contained sediment, while the other juvenile tank and the adult tank did not. This caused natural mortalities to occur earlier in the season and be observed during the study period. To verify that the tank environments closely resembled natural habitats, condition over time data were compared from in-situ sampling and tank observations in 2018 and 2019. There was no statistically significant difference in the natural decline in condition trend between in-situ and tank observations for juveniles; there was a significant difference for adults. However, this is likely due to sampling variability. Morphometrics and condition were taken for fish which died of starvation. This condition at mortality data was compared over time and across life stages to gauge if the threshold of survival was consistent through the overwintering period and within/between life stages. Results indicated that the threshold was consistent through time, suggesting that mortalities occurred due to fish reaching a “floor” of condition, regardless of if the death occurred early or late into the overwintering period. Condition at mortality differed significantly between life stages, with adults dying at a higher condition factor. Separate probability of mortality curves (eCDFs) were created for juveniles and adults to characterize the likelihood that an individual will reach mortality soon given a particular body condition. The threshold of survival was dictated by the condition at which fish have a 50% probability of mortality occurring soon. Juveniles were found to have a threshold at a condition factor (K) of 16.8, while the threshold for adults was 20. This threshold likely exists for other fishes and may be an important consideration for fisheries management. Juveniles of various fish species are disproportionately affected by winter mortality. Variable mortality could have large effects on abundance and year class strength. Since mortality rates differ by life stage and based on environmental conditions year to year, managers could reference mortality curves and oceanographic conditions to make better informed regulations which incorporate considerations for years with high mortality rates.