Lawson Gareth L.

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Lawson

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Gareth L.

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Improved parametrization of Antarctic krill target strength models

(Acoustical Society of America, 2006-01) Lawson, Gareth L. ; Wiebe, Peter H. ; Ashjian, Carin J. ; Chu, Dezhang ; Stanton, Timothy K.

There are historical discrepancies between empirical observations of Antarctic krill target strength and predictions using theoretical scattering models. These differences are addressed through improved understanding of key model parameters. The scattering process was modeled using the distorted-wave Born approximation, representing the shape of the animal as a bent and tapered cylinder. Recently published length-based regressions were used to constrain the sound speed and density contrasts between the animal and the surrounding seawater, rather than the earlier approach of using single values for all lengths. To constrain the parameter governing the orientation of the animal relative to the incident acoustic wave, direct measurements of the orientation of krill in situ were made with a video plankton recorder. In contrast to previous indirect and aquarium-based observations, krill were observed to orient themselves mostly horizontally. Averaging predicted scattering over the measured distribution of orientations resulted in predictions of target strength consistent with in situ measurements of target strength of large krill (mean length 40–43 mm) at four frequencies (43–420 kHz), but smaller than expected under the semi-empirical model traditionally used to estimate krill target strength.
Mesoscale eddies influence the movements of mature female white sharks in the Gulf Stream and Sargasso Sea

(Nature Publishing Group, 2018-05-09) Gaube, Peter ; Braun, Camrin D. ; Lawson, Gareth L. ; McGillicuddy, Dennis J. ; Penna, Alice Della ; Skomal, Gregory B. ; Fischer, Chris ; Thorrold, Simon R.

Satellite-tracking of mature white sharks (Carcharodon carcharias) has revealed open-ocean movements spanning months and covering tens of thousands of kilometers. But how are the energetic demands of these active apex predators met as they leave coastal areas with relatively high prey abundance to swim across the open ocean through waters often characterized as biological deserts? Here we investigate mesoscale oceanographic variability encountered by two white sharks as they moved through the Gulf Stream region and Sargasso Sea in the North Atlantic Ocean. In the vicinity of the Gulf Stream, the two mature female white sharks exhibited extensive use of the interiors of clockwise-rotating anticyclonic eddies, characterized by positive (warm) temperature anomalies. One tagged white shark was also equipped with an archival tag that indicated this individual made frequent dives to nearly 1,000 m in anticyclones, where it was presumably foraging on mesopelagic prey. We propose that warm temperature anomalies in anticyclones make prey more accessible and energetically profitable to adult white sharks in the Gulf Stream region by reducing the physiological costs of thermoregulation in cold water. The results presented here provide valuable new insight into open ocean habitat use by mature, female white sharks that may be applicable to other large pelagic predators.
Exposure to CO2 influences metabolism, calcification and gene expression of the thecosome pteropod Limacina retroversa

(The Company of Biologists, 2018-02-13) Maas, Amy E. ; Lawson, Gareth L. ; Bergan, Alexander J. ; Tarrant, Ann M.

Thecosomatous pteropods, a group of aragonite shell-bearing zooplankton, are becoming an important sentinel organism for understanding the influence of ocean acidification on pelagic organisms. These animals show vulnerability to changing carbonate chemistry conditions, are geographically widespread, and are both biogeochemically and trophically important. The objective of this study was to determine how increasing duration and severity of CO2 treatment influence the physiology of the thecosome Limacina retroversa, integrating both gene expression and organism-level (respiration and calcification) metrics. We exposed pteropods to over-saturated, near-saturated or under-saturated conditions and sampled individuals at 1, 3, 7, 14 and 21 days of exposure to test for the effect of duration. We found that calcification was affected by borderline and under-saturated conditions by week two, while respiration appeared to be more strongly influenced by an interaction between severity and duration of exposure, showing complex changes by one week of exposure. The organismal metrics were corroborated by specific gene expression responses, with increased expression of biomineralization-associated genes in the medium and high treatments throughout and complex changes in metabolic genes corresponding to both captivity and CO2 treatment. Genes associated with other physiological processes such as lipid metabolism, neural function and ion pumping had complex responses, influenced by both duration and severity. Beyond these responses, our findings detail the captivity effects for these pelagic organisms, providing information to contextualize the conclusions of previous studies, and emphasizing a need for better culturing protocols.
Changes in anthropogenic carbon storage in the Northeast Pacific in the last decade

(John Wiley & Sons, 2016-07-02) Chu, Sophie N. ; Wang, Zhaohui Aleck ; Doney, Scott C. ; Lawson, Gareth L. ; Hoering, Katherine A.

In order to understand the ocean's role as a sink for anthropogenic carbon dioxide (CO2), it is important to quantify changes in the amount of anthropogenic CO2 stored in the ocean interior over time. From August to September 2012, an ocean acidification cruise was conducted along a portion of the P17N transect (50°N 150°W to 33.5°N 135°W) in the Northeast Pacific. These measurements are compared with data from the previous occupation of this transect in 2001 to estimate the change in the anthropogenic CO2 inventory in the Northeast Pacific using an extended multiple linear regression (eMLR) approach. Maximum increases in the surface waters were 11 µmol kg−1 over 11 years near 50°N. Here, the penetration depth of anthropogenic CO2 only reached ∼300 m depth, whereas at 33.5°N, penetration depth reached ∼600 m. The average increase of the depth-integrated anthropogenic carbon inventory was 0.41 ± 0.12 mol m−2 yr−1 across the transect. Lower values down to 0.20 mol m−2 yr−1 were observed in the northern part of the transect near 50°N and increased up to 0.55 mol m−2 yr−1 toward 33.5°N. This increase in anthropogenic carbon in the upper ocean resulted in an average pH decrease of 0.002 ± 0.0003 pH units yr−1 and a 1.8 ± 0.4 m yr−1 shoaling rate of the aragonite saturation horizon. An average increase in apparent oxygen utilization of 13.4 ± 15.5 µmol kg−1 centered on isopycnal surface 26.6 kg m−3 from 2001 to 2012 was also observed.
Biogeographic variations in diel vertical migration determined from acoustic backscattering in the northwest Atlantic Ocean

(Elsevier, 2023-03-02) Wiebe, Peter H. ; Lavery, Andone C. ; Lawson, Gareth L.

Active acoustic scattering techniques, which capitalize on the fact that different kinds of organisms scatter sound differently as the frequency changes, are uniquely suited to synoptic studies of zooplankton and fish distributions. Acoustic systems are particularly useful for documenting diel vertical migrations (DVM). Measurements with a hull mounted echosounder operating at frequencies of 43, 120, 200, and 420 kHz were made near-continuously on a 26-day cruise in the Northwest Atlantic Ocean in August 2011. The distribution of backscattering in relation to changing environmental quantities along the cruise track enabled characterization of the rates and amplitudes of diel vertical migrations and acoustic indices of pelagic animal abundance in relation to hydrographic regimes. The most pervasive phenomenon observed acoustically was a regular DVM evident along a series of six study transects. In the mode waters (∼18 °C) of the Sargasso Sea, a non-migrating layer was observed around 200 m, which was not present in the other surveyed regions (Gulf Stream, GS/Labrador Sea loop, Flemish Cap, Slope Water). A deeper (>450 m) non-migrating layer existed throughout the survey region. Migration speeds varied between 361 m/hour in the Sargasso Sea and 72.5 m/hour in the GS/Labrador Sea Loop. Acoustic backscattering at 43 kHz in the upper 500 m was highest in the Sargasso Sea (day overall median = −79.5 dB; night median = −76.5 dB) and lower in the more northern stations (all the other regions day = −86.8 dB and night = −81.1 dB). Variations in the depth of the DSL at the sampled locations are not limited by subsurface oxygen levels in the Northwest Atlantic and Northern North Atlantic.•Acoustic backscattering layers differ between hydrographic regions in the NW Atlantic.•Diel Vertical Migration (DVM) is a dominant feature of the Northwestern Atlantic Ocean.•DVM varies in speed and distance between hydrographic regions in the NW Atlantic Ocean.•The Sargasso Sea has deeper and faster DVM than other regions in the NW Atlantic Ocean.
Reexamination of the species assignment of Diacavolinia pteropods using DNA barcoding

(Public Library of Science, 2013-01-15) Maas, Amy E. ; Blanco-Bercial, Leocadio ; Lawson, Gareth L.

Thecosome pteropods (Mollusca, Gastropoda) are an ecologically important, diverse, and ubiquitous group of holoplanktonic animals that are the focus of intense research interest due to their external aragonite shell and vulnerability to ocean acidification. Characterizing the response of these animals to low pH and other environmental stressors has been hampered by continued uncertainty in their taxonomic identification. An example of this confusion in species assignment is found in the genus Diacavolinia. All members of this genus were originally indentified as a single species, Cavolinia longirostris, but over the past fifty years the taxonomy has been revisited multiple times; currently the genus comprises 22 different species. This study examines five species of Diacavolinia, including four sampled in the Northeast Atlantic (78 individuals) and one from the Eastern tropical North Pacific (15 individuals). Diacavolina were identified to species based on morphological characteristics according to the current taxonomy, photographed, and then used to determine the sequence of the “DNA barcoding” region of the cytochrome c oxidase subunit I (COI). Specimens from the Atlantic, despite distinct differences in shell morphology, showed polyphyly and a genetic divergence of <3% (K2P distance) whereas the Pacific and Atlantic samples were more distant (~19%). Comparisons of Diacavolinia spp. with other Cavolinia spp. reveal larger distances (~24%). These results indicate that specimens from the Atlantic comprise a single monophyletic species and suggest possible species-level divergence between Atlantic and Pacific populations. The findings support the maintenance of Diacavolinia as a separate genus, yet emphasize the inadequacy of our current taxonomic understanding of pteropods. They highlight the need for accurate species identifications to support estimates of biodiversity, range extent and natural exposure of these planktonic calcifiers to environmental variability; furthermore, the apparent variation of the pteropods shell may have implications for our understanding of the species’ sensitivity to ocean acidification.
Exploiting signal processing approaches for broadband echosounders

(International Council for the Exploration of the Sea, 2017-08-28) Lavery, Andone C. ; Bassett, Christopher ; Lawson, Gareth L. ; Jech, J. Michael

Broadband echosounders, which transmit frequency-modulated pulses, increase the spectral characterization of targets relative to narrowband echosounders, which typically transmit single-frequency pulses, and also increase the range resolution through broadband matched-filter signal processing approaches. However, the increased range resolution does not necessarily lead to improved detection and characterization of targets close to boundaries due to the presence of undesirable signal processing side lobes. The standard approach to mitigating the impact of processing side lobes is to transmit tapered signals, which has the consequence of also reducing spectral information. To address this, different broadband signal processing approaches are explored using data collected in a large tank with both a Kongsberg–Simrad EK80 scientific echosounder with a combination of single- and split-beam transducers with nominal centre frequencies of 18, 38, 70, 120, 200, and 333 kHz, and with a single-beam custom-built echosounder spanning the frequency band from 130 to 195 kHz. It is shown that improved detection and characterization of targets close to boundaries can be achieved by using modified replica signals in the matched filter processing. An additional benefit to using broadband echosounders involves exploiting the frequency dependence of the beam pattern to calibrate single-beam broadband echosounders using an off-axis calibration sphere.
The effect of elevated carbon dioxide on the sinking and swimming of the shelled pteropod Limacina retroversa

( 2017-01) Bergan, Alexander J. ; Lawson, Gareth L. ; Maas, Amy E. ; Wang, Zhaohui Aleck

Shelled pteropods are planktonic molluscs that may be affected by ocean acidification. Limacina retroversa from the Gulf of Maine were used to investigate the impact of elevated carbon dioxide (CO2) on shell condition as well as swimming and sinking behaviours. Limacina retroversa were maintained at either ambient (ca. 400 μatm) or two levels of elevated CO2 (800 and 1200 μatm) for up to four weeks, and then examined for changes in shell transparency, sinking speed, and swimming behaviour assessed through a variety of metrics (e.g., speed, path tortuosity, wing beat frequency). After exposures to elevated CO2 for as little as four days, the pteropod shells were significantly darker and more opaque in the elevated CO2 treatments. Sinking speeds were significantly slower for pteropods exposed to medium and high CO2 in comparison to the ambient treatment. Swimming behaviour showed less clear patterns of response to treatment and duration of exposure, but overall, swimming did not appear to be hindered under elevated CO2. Sinking is used by L. retroversa for predator evasion, and altered speeds and increased visibility could increase the susceptibility of pteropods to predation.
Evidence of resource partitioning between humpback and minke whales around the western Antarctic Peninsula

( 2008-09-20) Friedlaender, Ari S. ; Lawson, Gareth L. ; Halpin, Patrick N.

For closely related sympatric species to coexist, they must differ to some degree in their ecological requirements or niches (e.g., diets) to avoid inter-specific competition. Baleen whales in the Antarctic feed primarily on krill, and the large sympatric pre-whaling community suggests resource partitioning among these species or a non-limiting prey resource. In order to examine ecological differences between sympatric humpback and minke whales around the Western Antarctic Peninsula, we made measurements of the physical environment, observations of whale distribution, and concurrent acoustic measurements of krill aggregations. Mantel’s tests and Classification and regression tree models indicate both similarities and differences in the spatial associations between humpback and minke whales, environmental features, and prey. The data suggest (1) similarities (proximity to shore) and differences (prey abundance versus deep water temperatures) in horizontal spatial distribution patterns, (2) unambiguous vertical resource partitioning with minke whales associating with deeper krill aggregations across a range of spatial scales, and (3) that interference competition between these two species is unlikely. These results add to the paucity of ecological knowledge relating baleen whales and their prey in the Antarctic and should be considered in conservation and management efforts for Southern Ocean cetaceans and ecosystems.
Pump it Up workshop report

(Woods Hole Oceanographic Institution, 2017-10-20) Buesseler, Ken O. ; Adams, Allan ; Bellingham, James G. ; Dever, Mathieu ; Edgcomb, Virginia P. ; Estapa, Margaret L. ; Frank, Alex ; Gallager, Scott M. ; Govindarajan, Annette F. ; Horner, Tristan J. ; Hunter, Jon ; Jakuba, Michael V. ; Kapit, Jason ; Katija, Kakani ; Lawson, Gareth L. ; Lu, Yuehan ; Mahadevan, Amala ; Nicholson, David P. ; Omand, Melissa M. ; Palevsky, Hilary I. ; Rauch, Chris ; Sosik, Heidi M. ; Ulmer, Kevin M. ; Wurgaft, Eyal ; Yoerger, Dana R.

A two-day workshop was conducted to trade ideas and brainstorm about how to advance our understanding of the ocean’s biological pump. The goal was to identify the most important scientific issues that are unresolved but might be addressed with new and future technological advances.
Transcriptome-wide analysis of the response of the thecosome pteropod Clio pyramidata to short-term CO2 exposure

( 2015-01) Maas, Amy E. ; Lawson, Gareth L. ; Tarrant, Ann M.

Thecosome pteropods, a group of calcifying holoplanktonic molluscs, have recently become a research focus due to their potential sensitivity to increased levels of anthropogenic dissolved CO2 in seawater and the accompanying ocean acidification. Some populations, however, already experience high CO2 in their natural distribution during diel vertical migrations. To achieve a better understanding of the mechanisms of pteropod calcification and physiological response to this sort of short duration CO2 exposure, we characterized the gene complement of Clio pyramidata, a cosmopolitan diel migratory thecosome, and investigated its transcriptomic response to experimentally manipulated CO2 conditions. Individuals were sampled from the Northwest Atlantic in the fall of 2011 and exposed to ambient conditions (~380 ppm) and elevated CO2 (~800 ppm, similar to levels experienced during a diel vertical migration) for ~10 hrs. Following this exposure the respiration rate of the individuals was measured. We then performed RNA-seq analysis, assembled the C. pyramidata transcriptome de novo, annotated the genes, and assessed the differential gene expression patterns in response to exposure to elevated CO2. Within the transcriptome, we identified homologs of genes with known roles in biomineralization in other molluscs, including perlucin, calmodulin, dermatopontin, calponin, and chitin synthases. Respiration rate was not affected by short-term exposure to CO2. Gene expression varied greatly among individuals, and comparison between treatments indicated that C. pyramidata down-regulated a small number of genes associated with aerobic metabolism and up-regulated genes that may be associated with biomineralization, particularly collagens and C- type lectins. These results provide initial insight into the effects of short term CO2 exposure on these important planktonic open-ocean calcifiers, pairing respiration rate and the gene expression level of response, and reveal candidate genes for future ecophysiological, biomaterial and phylogenetic studies.
The metabolic response of thecosome pteropods from the North Atlantic and North Pacific oceans to high CO2 and low O2

(Copernicus Publications on behalf of the European Geosciences Union, 2016-11-17) Maas, Amy E. ; Lawson, Gareth L. ; Wang, Zhaohui Aleck

As anthropogenic activities directly and indirectly increase carbon dioxide (CO2) and decrease oxygen (O2) concentrations in the ocean system, it becomes important to understand how different populations of marine animals will respond. Water that is naturally low in pH, with a high concentration of carbon dioxide (hypercapnia) and a low concentration of oxygen, occurs at shallow depths (200–500 m) in the North Pacific Ocean, whereas similar conditions are absent throughout the upper water column in the North Atlantic. This contrasting hydrography provides a natural experiment to explore whether differences in environment cause populations of cosmopolitan pelagic calcifiers, specifically the aragonitic-shelled pteropods, to have a different physiological response when exposed to hypercapnia and low O2. Using closed-chamber end-point respiration experiments, eight species of pteropods from the two ocean basins were exposed to high CO2 ( ∼  800 µatm) while six species were also exposed to moderately low O2 (48 % saturated, or ∼  130 µmol kg−1) and a combined treatment of low O2/high CO2. None of the species tested showed a change in metabolic rate in response to high CO2 alone. Of those species tested for an effect of O2, only Limacina retroversa from the Atlantic showed a response to the combined treatment, resulting in a reduction in metabolic rate. Our results suggest that pteropods have mechanisms for coping with short-term CO2 exposure and that there can be interactive effects between stressors on the physiology of these open ocean organisms that correlate with natural exposure to low O2 and high CO2. These are considerations that should be taken into account in projections of organismal sensitivity to future ocean conditions.
Estimation of biological parameters of marine organisms using linear and nonlinear acoustic scattering model-based inversion methods

(Acoustical Society of America, 2016-05-19) Chu, Dezhang ; Lawson, Gareth L. ; Wiebe, Peter H.

The linear inversion commonly used in fisheries and zooplankton acoustics assumes a constant inversion kernel and ignores the uncertainties associated with the shape and behavior of the scattering targets, as well as other relevant animal parameters. Here, errors of the linear inversion due to uncertainty associated with the inversion kernel are quantified. A scattering model-based nonlinear inversion method is presented that takes into account the nonlinearity of the inverse problem and is able to estimate simultaneously animal abundance and the parameters associated with the scattering model inherent to the kernel. It uses sophisticated scattering models to estimate first, the abundance, and second, the relevant shape and behavioral parameters of the target organisms. Numerical simulations demonstrate that the abundance, size, and behavior (tilt angle) parameters of marine animals (fish or zooplankton) can be accurately inferred from the inversion by using multi-frequency acoustic data. The influence of the singularity and uncertainty in the inversion kernel on the inversion results can be mitigated by examining the singular values for linear inverse problems and employing a non-linear inversion involving a scattering model-based kernel.
Wideband (15–260 kHz) acoustic volume backscattering spectra of Northern krill (Meganyctiphanes norvegica) and butterfish (Peprilus triacanthus)

(International Council for the Exploration of the Sea, 2017-08-28) Jech, J. Michael ; Lawson, Gareth L. ; Lavery, Andone C.

Measurements of acoustic backscatter made over a wide frequency band have the potential for improved classification relative to traditional narrowband methods, by characterizing more fully the frequency response of scatterers. In January 2014, five wideband transceivers [Simrad EK80 Wideband Transceivers (WBTs)] and split-beam transducers with nominal centre frequencies of 18, 38, 70, 120, and 200 kHz were used to collect acoustic data spanning a nearly continuous 15–260 kHz bandwidth. The acoustic samples were from ca. 2 m below the surface to the seabed in an area along the US continental shelf break. Bottom trawls and zooplankton nets were also used to sample scatterers contributing to selected features of the acoustic backscatter. Measurements of frequency-dependent volume backscattering strength (i.e. volume backscattering spectra) from aggregations of euphausiids (mostly Northern krill, Meganyctiphanes norvegica) clearly resolved the transition from Rayleigh to geometric scattering, consistent with modelled backscatter from the type and length of animals sampled with bongo nets. Volume backscattering spectra from aggregations dominated by butterfish (Peprilus triacanthus) revealed a frequency response that was suggestive of superimposed scattering by soft tissue and bone. Backscatter predicted by Kirchhoff ray mode models of butterfish corresponded to trends in the measured spectra, supporting the assumption that acoustic scattering by butterfish is dominated by soft tissue and vertebrae.
Determining dominant scatterers of sound in mixed zooplankton populations

(Acoustical Society of America, 2007-12) Lavery, Andone C. ; Wiebe, Peter H. ; Stanton, Timothy K. ; Lawson, Gareth L. ; Benfield, Mark C. ; Copley, Nancy J.

High-frequency acoustic scattering techniques have been used to investigate dominant scatterers in mixed zooplankton populations. Volume backscattering was measured in the Gulf of Maine at 43, 120, 200, and 420 kHz. Zooplankton composition and size were determined using net and video sampling techniques, and water properties were determined using conductivity, temperature, and depth sensors. Dominant scatterers have been identified using recently developed scattering models for zooplankton and microstructure. Microstructure generally did not contribute to the scattering. At certain locations, gas-bearing zooplankton, that account for a small fraction of the total abundance and biomass, dominated the scattering at all frequencies. At these locations, acoustically inferred size agreed well with size determined from the net samples. Significant differences between the acoustic, net, and video estimates of abundance for these zooplankton are most likely due to limitations of the net and video techniques. No other type of biological scatterer ever dominated the scattering at all frequencies. Copepods, fluid-like zooplankton that account for most of the abundance and biomass, dominated at select locations only at the highest frequencies. At these locations, acoustically inferred abundance agreed well with net and video estimates. A general approach for the difficult problem of interpreting high-frequency acoustic scattering in mixed zooplankton populations is described.
Does predation control the diapausing stock of Calanus finmarchicus in the Gulf of Maine?

(Elsevier, 2022-07-29) Wiebe, Peter ; Baumgartner, Mark F. ; Copley, Nancy ; Lawson, Gareth L. ; Davis, Cabell S. ; Ji, Rubao ; Greene, Charles H.

The variability of zooplankton populations is controlled by external and internal forcing, with the former being principally large-scale changes in circulation, and the latter being driven by in situ growth, competition, and predation. Assessing the relative importance of these forcings is challenging and requires analyses of multifaceted observational data. As part of the U.S. GLOBEC Georges Bank program, a series of cruises were conducted in fall 1997, 1998, and 1999 to survey diapausing populations of Calanus finmarchicus and their predators in Wilkinson, Jordan, and Georges Basins of the Gulf of Maine. Station and underway sampling were conducted using net (1 m2 MOCNESS) and bioacoustic (BIOMAPER-II) systems, respectively, to acquire vertically stratified data for zooplankton biomass, taxonomic, size, and life-stage composition, together with associated environmental data. The results show that the autumn diapausing C. finmarchicus abundance was much lower in 1998 than in 1997 or 1999, even though the overall zooplankton biomass levels were comparable between the three years. The size frequency distribution of the diapausing individuals had a bi-modal pattern in 1997 and 1999, but a single mode in 1998, indicating the demise of an early cohort of the diapausing stock. The relative biomass and computed energy demand of potential invertebrate predators (euphausiids, decapods, medusae, and siphonophores) was found to be higher in 1998 and could account for the missing C. finmarchicus cohort. Evidence collected from this study supports the hypothesis that local predation has the potential to control the diapausing stock of C. finmarchicus in the Gulf of Maine.
Whale distribution in relation to prey abundance and oceanographic processes in shelf waters of the Western Antarctic Peninsula

(Inter-Research, 2006-07-18) Friedlaender, Ari S. ; Halpin, Patrick N. ; Qian, Song S. ; Lawson, Gareth L. ; Wiebe, Peter H. ; Thiele, Deb ; Read, Andrew J.

The Western Antarctic Peninsula (WAP) is a biologically rich area supporting large standing stocks of krill and top predators (including whales, seals and seabirds). Physical forcing greatly affects productivity, recruitment, survival and distribution of krill in this area. In turn, such interactions are likely to affect the distribution of baleen whales. The Southern Ocean GLOBEC research program aims to explore the relationships and interactions between the environment, krill and predators around Marguerite Bay (WAP) in autumn 2001 and 2002. Bathymetric and environmental variables including acoustic backscattering as an indicator of prey abundance were used to model whale distribution patterns. We used an iterative approach employing (1) classification and regression tree (CART) models to identify oceanographic and ecological variables contributing to variability in humpback Megaptera novaeangliae and minke Balaenoptera acutorstrata whale distribution, and (2) generalized additive models (GAMs) to elucidate functional ecological relationships between these variables and whale distribution. The CART models indicated that the cetacean distribution was tightly coupled with zooplankton acoustic volume backscatter in the upper (25 to 100 m), and middle (100 to 300 m) portions of the water column. Whale distribution was also related to distance from the ice edge and bathymetric slope. The GAMs indicated a persistent, strong, positive relationship between increasing zooplankton volume and whale relative abundance. Furthermore, there was a lower limit for averaged acoustic volume backscatter of zooplankton below which the relationship between whales and prey was not significant. The GAMs also supported an annual relationship between whale distribution, distance from the ice edge and bathymetric slope, suggesting that these are important features for aggregating prey. Our results demonstrate that during the 2 yr study, whales were consistently and predictably associated with the distribution of zooplankton. Thus, humpback and minke whales may be able to locate physical features and oceanographic processes that enhance prey aggregation.
Life cycle and early development of the thecosomatous pteropod Limacina retroversa in the Gulf of Maine, including the effect of elevated CO2 levels

( 2015-10) Thabet, Ali A. ; Maas, Amy E. ; Lawson, Gareth L. ; Tarrant, Ann M.

Thecosome pteropods are pelagic molluscs with aragonitic shells. They are considered to be especially vulnerable among plankton to ocean acidification (OA), but to recognize changes due to anthropogenic forcing a baseline understanding of their life history is needed. In the present study, adult Limacina retroversa were collected on five cruises from multiple sites in the Gulf of Maine (between 42° 22.1’–42° 0.0’ N and 69° 42.6’–70° 15.4’ W; water depths of ca. 45–260 m) from October 2013−November 2014. They were maintained in the laboratory under continuous light at 8° C. There was evidence of year-round reproduction and an individual life span in the laboratory of 6 months. Eggs laid in captivity were observed throughout development. Hatching occurred after 3 days, the veliger stage was reached after 6−7 days, and metamorphosis to the juvenile stage was after ~ 1 month. Reproductive individuals were first observed after 3 months. Calcein staining of embryos revealed calcium storage beginning in the late gastrula stage. Staining was observed in the shell gland, shell field, mantle, and shell margin in later stages. Exposure of two batches of larvae at the gastrula stage to elevated CO2 levels (800 and 1200 ppm) resulted in significantly increased mortality in comparison with individuals raised under ambient (~400 ppm) conditions and a developmental delay in the 1200 ppm treatment compared with the ambient and 800 ppm treatments.
Atlantic bluefin tuna : a novel multistock spatial model for asessing population biomass

(Public Library of Science, 2011-12-09) Taylor, Nathan G. ; McAllister, Murdoch K. ; Lawson, Gareth L. ; Carruthers, Tom ; Block, Barbara A.

Atlantic bluefin tuna (Thunnus thynnus) is considered to be overfished, but the status of its populations has been debated, partly because of uncertainties regarding the effects of mixing on fishing grounds. A better understanding of spatial structure and mixing may help fisheries managers to successfully rebuild populations to sustainable levels while maximizing catches. We formulate a new seasonally and spatially explicit fisheries model that is fitted to conventional and electronic tag data, historic catch-at-age reconstructions, and otolith microchemistry stock-composition data to improve the capacity to assess past, current, and future population sizes of Atlantic bluefin tuna. We apply the model to estimate spatial and temporal mixing of the eastern (Mediterranean) and western (Gulf of Mexico) populations, and to reconstruct abundances from 1950 to 2008. We show that western and eastern populations have been reduced to 17% and 33%, respectively, of 1950 spawning stock biomass levels. Overfishing to below the biomass that produces maximum sustainable yield occurred in the 1960s and the late 1990s for western and eastern populations, respectively. The model predicts that mixing depends on season, ontogeny, and location, and is highest in the western Atlantic. Assuming that future catches are zero, western and eastern populations are predicted to recover to levels at maximum sustainable yield by 2025 and 2015, respectively. However, the western population will not recover with catches of 1750 and 12,900 tonnes (the “rebuilding quotas”) in the western and eastern Atlantic, respectively, with or without closures in the Gulf of Mexico. If future catches are double the rebuilding quotas, then rebuilding of both populations will be compromised. If fishing were to continue in the eastern Atlantic at the unregulated levels of 2007, both stocks would continue to decline. Since populations mix on North Atlantic foraging grounds, successful rebuilding policies will benefit from trans-Atlantic cooperation.
Accounting for seasonal and composition-related variability in acoustic material properties in estimating copepod and krill target strength

(Wiley, 2019-11-06) Sakinan, Serdar ; Lawson, Gareth L. ; Wiebe, Peter ; Copley, Nancy

Estimation of abundance or biomass, using acoustic techniques requires knowledge of the frequency dependent acoustic backscatter characteristics, or target strength, of organisms. Target strength of zooplankton is typically estimated from physics‐based models that involve multiple parameters, notably including the acoustic material properties (i.e., the contrasts in density and sound speed between the animal and surrounding seawater). In this work, variability in the acoustic material properties of two zooplankton species in the Gulf of Maine, the copepod (Calanus finmarchicus) and krill (Meganyctiphanes norvegica), was investigated relative to changing season as well as, for the copepod, temperature and depth. Increases in the density and sound speed contrasts of these species from fall to spring were observed. Target strength predictions based on these measurements varied between fall and spring by 2‐3 dB in krill. Measurements were also conducted on C. finmarchicus lipid extract at changing temperature and pressure. The density contrast of the extract varied negatively with temperature, while the sound speed contrast changed by more than 10 % over the temperature and pressure ranges that the organism expected to occupy. C. finmarchicus target strength predictions showed that the combined effect of temperature and pressure can be significant (more than 10 dB) due to the varying response of lipids. The large vertical migration ranges and lipid accumulation characteristics of these species (e.g., the diapause behaviour of Calanus copepods) suggest that it is necessary for seasonal and environmental variability in material properties to be taken into account to achieve reliable measurements.

Lawson Gareth L.

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