Patrick H. Ressler

Hydrographic and acoustic evidence for enhanced plankton stocks in a small cyclone in the northeastern Gulf of Mexico

Mesoscale eddies (diameters of hundreds of km) have been shown to influence plankton production as well as the distribution of seabirds and marine mammals in the Gulf of Mexico. Smaller circulation features (eddies with diameters of tens of km) may have similar effects. We show that a small, sub-mesoscale cyclone located on the continental shelf and slope in the northeastern Gulf of Mexico during November 1997 was an area of enhanced nutrients, chlorophyll, and acoustic volume backscattering strength ðSvÞ: Nitrate concentrations at mid-depth in the euphotic zone were as high as 2mM, surface chlorophyll exceeded 1m gl � 1 , and Sv was as much as 15 dB referenced to 1 m � 1 4p � 1 greater within this feature than in surrounding waters. Since Sv at 153 kHz is a proxy for the abundance of sound-scattering mesozooplankton and micronekton, we infer that this cyclone was locally enriched in biomass of these organisms. As with mesoscale eddies, smaller cyclones could also function as patches of favorable habitat for higher-trophic-level organisms and their prey. r 2003 Elsevier Science Ltd. All rights reserved.

Multifrequency species classification of acoustic-trawl survey data using semi-supervised learning with class discovery

Acoustic surveys often use multifrequency backscatter to estimate fish and plankton abundance. Direct samples are used to validate species classification of acoustic backscatter, but samples may be sparse or unavailable. A generalized Gaussian mixture model was developed to classify multifrequency acoustic backscatter when not all species classes are known. The classification, based on semi-supervised learning with class discovery, was applied to data collected in the eastern Bering Sea during summers 2004, 2007, and 2008. Walleye pollock, euphausiids, and two other major classes occurring in the upper water column were identified.

Acoustic vessel-of-opportunity (AVO) index for midwater Bering Sea walleye pollock, 2010-2011

An acoustic vessel of opportunity (AVO) index for midwater walleye pollock (Gadus chalcogrammus) has been estimated since 2006 using backscatter information collected during the annual bottom trawl (BT) survey. AVO index estimates for summer 2014 and 2015 are reported here. The 2014 AVO index increased 29% from the 2013 index value, and 36% from 2012. The 2015 AVO index increased slightly (6%) from 2014. Both estimates (2014, 2015) exceeded all earlier time series estimates (2006-2013) based on non-overlapping 95% confidence intervals. Most pollock backscatter appeared to be distributed broadly across the shelf between 50 and 200 m isobaths in 2014 and 2015. The percentage of pollock backscatter east of the Pribilof Islands (east of 170○ W longitude) in the AVO index was 24% in 2014 and 25% in 2015. This was similar to the percentage in 2013 (26%), but much greater than reported for summers 2010-2012 (range 4-9%). This implies that there has been more midwater pollock biomass east of the Pribilof Islands in recent years. Comparison of the AVO index and AT survey time series continues to show a strong correlation (r2 = 0.90, p = 0.0011). Midwater hauls were conducted to sample midwater pollock aggregations during the 2014 (n = 31) and 2015 (n = 32) BT surveys to investigate the feasibility of using these hauls to convert the AVO backscatter index to abundance at length or age. Some portions of the AVO index area were not sampled by these hauls in both years. Preliminary analyses of these haul data (ability to target and catch pollock, catch composition, and length-frequency comparisons) showed 1) hauls targeted appropriate fish layers and were dominated by pollock, 2) bottom trawls and midwater trawls caught pollock of different length compositions and 3) length modes in midwater hauls from BT and AT surveys were similar, but occurred in different proportions even when restricted to the same subarea. Due to a number of factors including logistical and iii

Results of the acoustic-trawl survey of Walleye pollock (Gadus chalcogrammus) in the Gulf of Alaska, June - August 2013 (DY2013-07)

Scientists from the Midwater Assessment and Conservation Engineering (MACE) Program of the Alaska Fisheries Science Centers (AFSC) Resource Assessment and Conservation Engineering (RACE) Division conducted an acoustic-trawl (AT) survey of the Gulf of Alaska (GOA) shelf to estimate the distribution and abundance of walleye pollock (Gadus chalcogrammus) in summer 2017. Previous surveys of the GOA have also been conducted by the MACE program during the summers of 2003, 2005, 2011, 2013, and 2015. The 2017 survey covered the shelf from the Islands of Four Mountains to Yakutat Trough including many bays and troughs. Surface water temperatures across the GOA shelf averaged 11.6° C, overall, approximately 0.6° C cooler than in 2015 (mean 12.2° C) and 1.0° C warmer than in 2013 (mean 10.6° C), which were the only other surveys with comparable coverage. Large aggregations of age-0 pollock were observed throughout the survey but especially in the Shumagin Islands and Shelikof Strait areas. The estimated amounts of age-1+ pollock for the entire surveyed area were 2.64 billion fish weighing 1,341,973 metric tons (t). The majority of the pollock biomass was observed on the continental shelf (84%), Shelikof Strait (5%), near Mitrofania Island (3%), and east of Kodiak Island in Chiniak (2%) and Barnabas Troughs (4%). The majority (86%) of the biomass in the survey area consisted of age-5 fish from the 2012 year class. Compared to previous summer GOA surveys, fish weight at length was similar, but length at age was slightly smaller. Backscatter was attributed to species other than pollock when trawl verification, differentiation based on backscatter frequency response, or other methods allowed. Abundance and biomass estimates were calculated for Pacific ocean perch (Sebastes alutus; 305.9 million fish weighing 172,388 t), and backscatter distribution and abundance relative to previous surveys was estimated for euphausiids (primarily consisting of Thysanoessa inermis, but also including T. spinifera, T. raschii, and Euphausia pacifica).

Acoustic estimates of zooplankton and micronekton biomass using an ADCP

A calibrated 153‐kHz narrow‐band ADCP (Acoustic Doppler Current Profiler) has been used to collect acoustic backscatter intensity (Sv) data during several cruises in the Gulf of Mexico. Data have been gathered both while on station and while underway along transects through different hydrographic regimes, enabling an examination of both temporal and spatial trends in backscatter. In addition, zooplankton and micronekton stocks were intensively sampled with a 1 m2 MOCNESS (Multiple Opening‐Closing Net Environmental Sensing System). Empirical correlations between spatial and temporal variations in Sv and in standing stocks of zooplankton/micronekton will be presented. Sv measured with an ADCP may be useful as an index of zooplankton and micronekton biomass. The location and abundance of these food stocks are hypothesized to be important in determining the distribution and abundance of marine cetaceans in the Gulf of Mexico. In addition, Sv‐derived estimates of such stocks may allow inferences about seconda...

Using acoustic data from fishing vessels to estimate walleye pollock abundance in the eastern Bering Sea.

Eastern Bering Sea walleye pollock (Theragra chalcogramma) supports one of the world’s largest fisheries. Due to pollock’s high recruitment variability and relatively short life span, timely and accurate abundance indices are needed for fisheries management. Annual bottom trawl (BT) surveys track the demersal portion of the pollock population using chartered commercial fishing vessels, while biennial acoustic‐trawl (AT) surveys use NOAA research vessels to track the younger, midwater portion of the population. More frequent, annual indices of walleye pollock stock size may be obtained using acoustic backscatter data collected from the commercial fishing vessels conducting the BT survey at relatively low cost. A retrospective analysis of 38 kHz, AT survey data (1999–2004) identified a suitable index area to track midwater pollock abundance. The BT survey acoustic data (2006–2009) collected in that area tracked both the AT survey pollock abundance and the large‐scale pollock distribution patterns. This stud...

Euphausiids in the Bering Sea: better ecosystem information through acoustics.

Better information on the distribution and abundance of euphausiids (Thysanoessa spp.), a key group of crustacean zooplankton, meets a specific assessment need in the Bering Sea ecosystem. Multifrequency acoustic classification, backscatter modeling, and net capture were recently used to develop, apply, and validate a method of surveying euphausiid distribution and abundance during regular acoustic‐trawl surveys of walleye pollock (Theragra chalcogramma), an important commercial fish stock. These observations of euphausiids have both ecological and management implications. Summer surveys indicate that pollock predation may control euphausiid abundance: the stocks are inversely correlated in space and time, and estimated predation by pollock is sufficient to influence the euphausiid standing stock. Spring observations show that euphausiids and pollock are spatially segregated by ice cover and water temperature, which might mediate predation by pollock. This information on euphausiid abundance and distribut...

Improved assessment of Arctic and Antarctic zooplankton: From acoustic scattering models to a better understanding of polar ecosystems.

Polar regions contain some of the most productive (and in some cases unspoiled) ecosystems on the planet. Zooplankton, particularly krill, are the preferred prey for numerous fish, bird, and mammal species including animals that are endangered, commercially fished, or ecologically important. Acoustic echosounders provide an excellent method for assessing the abundance and distribution of zooplankton over large areas (> 100 s km2) through ship‐based surveys and with high‐temporal and spatial resolution. However, in order to accurately convert acoustic backscatter into a biologically meaningful measure (e.g., animal abundance, biomass, and calories), validated scattering models with well‐understood input parameters are needed. This talk will present data from both Antarctic and Arctic (Bering Sea) zooplankton with regard to improvement of scattering models and the range and variability of model inputs for krill and other zooplankton, implications of these changes on acoustic estimates of zooplankton biomass...

Density and soundspeed measurements of various Bering Sea zooplankton.

Walleye pollock is a commercially and ecologically important species in the Bering Sea. Assessment of pollock stocks is conducted regularly using a combination of trawl net sampling and multiple frequency active acoustic echosounder surveys. In addition to backscatter from the species of interest, other marine life including several types of zooplankton and nekton can contribute substantial amounts of acoustic backscatter energy which can provide information regarding the abundance and distribution of these animals as well. Acoustic scattering models are used to estimate both the amount and frequency response of the acoustic energy scattered by the zooplankton, but these models require knowledge of various properties that describe the acoustically important characteristics of the zooplankton. We collected live zooplankton from the Bering Sea during June and July 2008. The morphology (spatial dimensions) and material properties (density and sound speed) of individual and groups of zooplankton (and some nek...

Optimal volume backscattering thresholds for echo integration

Echo integration is a commonly used technique for assessment of fish stocks. In echo integration, a frequently used method to increase the signal‐to‐noise ratio is to ignore data below a set volume backscattering threshold. Although this form of thresholding is common, objective and replicable techniques for choosing this threshold are rarely used. Two opposing goals come into play when choosing an optimal threshold for estimating fish biomass using echo integration: maximizing the energy from backscatterers of interest, while simultaneously minimizing the energy from backscatterers not of interest. Both empirical and modeling techniques for choosing optimal thresholds were demonstrated using data from the 2003 integrated acoustic and trawl survey of Pacific hake. Empirical techniques were based on the receiver operating characteristic (ROC) curve. An ROC curve is a graphical plot of the number of true positives versus false positives for a binary classification system as the discrimination threshold is v...