No abstract

Target strength model inputs including morphometry, material properties, lipid composition, and in situ orientations were measured for sub-Arctic krill (Euphausia pacifica, Thysanoessa spinifera, T. inermis, and T. raschii) in the eastern Bering Sea (EBS, 2016) and Gulf of Alaska (GOA, 2017). Inter-species and -regional animal lengths were significantly different (F1,680 = 114.10, p < 0.01), while animal shape was consistent for all species measured. The polar lipid phosphatidycholine was the dominant lipid, comprising 86 ± 16% (mean ± SD) and 56 ± 22% of total lipid mass in GOA and EBS krill, respectively. Krill density contrasts varied by species and region rather than with morphometry, lipid composition, or local chla fluorescence. Mean in situ krill orientation was 1 ± 31°, with 25% of observed krill within ±5° of broadside incidence. Modelled target strength sensitivity was frequency independent for variations in material properties but was primarily sensitive to morphometry and orientation at lower (38 kHz) and higher (200 kHz) frequencies, respectively. Measured variability in material properties corresponded to an order of magnitude difference in acoustic estimates of biomass at 120 kHz. These results provide important inputs and constraints for acoustic scattering models of ecologically important sub-Arctic krill species.

During the 14–24 May 2019 leg of the NOAA juvenile rockfish survey, zooplankton and small nekton were sampled off the coast of Northern California using a midwater trawl. This talk will focus on three euphausiid species (E. pacifica, T. spinifera, N. difficilis) which were present in most catches. Krill morphology (size and shape) and animal density relative to seawater were measured for 175 live (or very recently expired) individuals. Roughly 150 individual krill were frozen and brought back to land for mass measurements. Soundspeed relative to seawater was measured for 15 mixed-species aggregations of krill. Broadband (38–73 and 130–210 kHz) and narrowband (38, 50, 70, 120, 150, and 200 kHz) backscatter measurements were made on 32 live (or very recently expired) individual krill. These empirical measurements were then compared to predictive backscatter measurements using scattering model parameters measured for those individuals (when available). Differences between predicted and measured backscatter were generally small, although there were individual krill where measurements at some frequencies varied significantly (i.e. more than 3dB). Understanding the variability in scattering model inputs among and within these species, and the accuracy of scattering model predictions provides context (i.e. uncertainty estimates) for acoustic measures (or indices) of krill biomass.

1. The management of migratory taxa relies on the knowledge of their movements.Among them, ontogenetic habitat shift, from nurseries to adult habitats, is a behavioural trait shared across marine taxa allowing resource partitioning between life stages and reducing predation risk. As this movement is consistent over time, characterizing its timing is critical to implement efficient management plans, notably in coastal areas to mitigate the impact of fisheries on juvenile stocks.2. In the Mexican Pacific, habitat use of the smooth hammerhead shark (Sphyrna zygaena) is poorly described, while the species is heavily harvested. Given the large uncertainties associated with the timing of out-migration from coastal nursery grounds to offshore waters prior to reproductive maturity, a more precise assessment of smooth hammerhead shark movements is needed.3. Photochemical degradation of mercury imparts mass-independent isotope fractionation (Δ 199 Hg) which can be used to discriminate between neonate coastal shallow habitats and the offshore deep foraging patterns of late juveniles. Here, we present the application of muscle Δ 199 Hg as molecular clocks to predict the timing of ontogenetic habitat shifts by smooth hammerhead sharks, based on their isotopic compositions at the initial and arrival habitats and on muscle isotopic turnover rate. 4. We observed decreases in Δ 199 Hg values with shark body length, reflecting increasing reliance on offshore mesopelagic prey with age. Coastal residency estimates indicated that smooth hammerhead sharks utilize coastal resources for 2 years prior to offshore migration, suggesting a prolonged residency in these ecosystems. 85 0.31 ± 0.20 115 (112-118) 390 (382-396)

Active acoustic surveys provide fine-scale measurements over broad spatial and temporal scales of organism abundance in the water column which can be useful as ecosystem indicators. As part of a long-term time series monitoring program, we conducted seasonal surveys within the New York Bight beginning in 2017. The survey comprises eight transects that extend from near-shore to the continental shelf break, and include multiple stations within each transect where net tows and CTD casts are conducted. Acoustic backscatter measurements at 38, 70, 120, and 200 kHz from the water column were integrated (NASC, m2 nmi−2) to examine the distribution and relative abundance of pelagic scatterers. Generally, NASC was highest during the summer and fall surveys, which corresponds to expected seasonal migrations of fish and squid in this area. Winter NASC distributions were highest in waters deeper than 50 m, but increased nearshore during the summer and fall surveys. Backscatter also appeared to be associated with bathymetric features, such as the shelfbreak and the Hudson Canyon, and in some cases with hydrographic fronts and gradients. These data are the first steps in a longer-term monitoring program of pelagic organisms in the New York Bight.