Pacific Sand Lance Energy Density, Length, and Age, Prince William Sound, Alaska, 2012-2016

Fish total length was measured to the nearest mm from either freshly captured individuals at sea (TLField_mm) or thawed individuals in the laboratory (TLLab_mm). Length was measured for thawed fish in the laboratory for all years except 2015, when lengths were only measured in the field. In 2016, lengths were measured in the field and laboratory for each fish and the data were used to develop a linear regression equation to convert measurements between freshly captured and thawed sand lance (TLfield_mm = 1.03 x TLLab_mm + 1.58; r2= 0.97, P <0.0001). Age was assigned by counting translucent annular rings on sagittal otoliths of sand lance by two independent, blind readers. In the event of disagreement in age assignment, readers re-examined an otolith jointly and assigned a consensus age for analysis. Otoliths were extracted, dried, and examined under reflected light using a Leica M60 dissection microscope. Under reflected light, translucent zones appear dark and opaque zones appear white (Figure 2). Translucent bands forming on the otolith edge were considered incomplete, assuming a January 1 birth date for sand lance (Robards et al. 2002). We assumed a consistent spawn and hatch timing among cohorts such that little variation (~1 month) in mean age exists within each age class across years that resulted in consistent ages within age class assignments. Digital images of each otolith were captured using a Leica DFC425 digital camera. To quantify the difference in otolith appearance among capture years, the “plus” growth was measured for the same subset of age-1 sand lance randomly selected for energy density analyses (n = 50, see below). The plus growth represents the growth that occurred within the spring-summer of collection and was measured from the outside edge of the translucent zone to the outside edge of the otolith along the longest axis from the nucleus to the edge. Otolith growth is strongly correlated to somatic size within fish species (Campana & Thorrold 2001). Energy density and whole body energy were estimated using bomb calorimetry on a subset of samples following otolith removal and age assignment. Ten individuals were randomly selected within each primary age class (age-0, age-1) and collection year for energy density analysis (n = 100). Whole fish were freeze-dried until weight stabilized and no moisture was apparent (approximately 48 hours). Dry mass was recorded to the nearest 0.0001 g. Dried fish were homogenized using a mortar and pestle and a pellet was pressed from a subsample and weighed immediately. A semimicro Parr 6725 calorimeter was used to measure energy density. Benzoic acid standards and duplicate tissue samples were used to evaluate precision. Energy density was reported per unit dry mass (kJ g-1 dry mass) as wet mass measurements likely introduce a desiccation bias that can mask the true biological relationships (Montevecchi & Piatt 1987, Hislop et al. 1991, Van Pelt et al. 1997, Ball et al. 2007). Whole body energy (kJ fish -1) was estimated by multiplying energy density (kJ g-1 dry mass) by dry mass (g). Whole body energy combines changes in energy density (e.g., energy storage) and size (e.g., somatic growth) and provides a metric of energy allocated to storage and growth. Moreover, whole body energy is an important index of nutritional value for sand lance predators, particular those who capture fish one at a time, as many seabirds do (Wanless et al. 2018).

All Pacific sand lance (Ammodytes personatus) were captured in July near the annual peak of body condition and lipid accumulation for sand lance in southcentral Alaska. The timing of peak body condition is determined by a life history that includes fall spawning and winter hatch, and coincides with the peak abundance of their primary prey, zooplankton. Our assumption of minimal (< 1 mo) interannual variation in the peak timing of sand lance body condition is supported by the consistent seasonality in the timing of peaks for chlorophyll-a concentrations and zooplankton abundances. Differences between sexes should be minimal during July. We collected sand lance for age composition and body conditionnutritional value using purse seine, beach seine, herring jig, cast net, dip net, and gill net. The purse seine was deployed from a skiff, and was 46.9 m long and 6.1 m deep, with a mesh size of 32 mm and 3.2 mm. The beach seine was set parallel to shore from a skiff and retrieved from the beach and was 37 m long with diminishing mesh size from 28 mm at the wings to 5 mm at the center. Herring jigs (hook size 4, 6, 8 and 10), cast nets (mesh size 6.4 mm and 19.1 mm), long-handled dip nets (mesh size 5 mm) and gill nets (18.2 m x 4.9 m with variable mesh panels of 6.4 mm, 7.9 mm, and 9.5 mm square mesh) were used infrequently to sample small, shallow schools.