Hydroacoustic differentiation of adult Atlantic salmon and aquatic macrophytes in the River Wye,Wales

Split-beam hydroacoustic techniques have been used to enumerate adult Atlantic salmon (Salmo salar) passage on the River Wye since 1994. Aggregations of aquatic macrophytes, principally Ranunculus fluitans, are seasonally present at the monitoring site. At high densities, these macrophytes return target strength (TS) values similar to those of adult salmon. Target direction-of-movement information readily resolves upstream-migrant salmon from static- or downstream-traveling macrophytes. However, Atlantic salmon are iteroparous, and downstream-migrant kelts must be factored into the acoustic estimates. Net upstream target movement and/or TS were not reliable indicators of salmon passage at the River Wye site when aquatic macrophyte aggregations were present. In 1995, a subset of data (n = 71) was selected from the ongoing monitoring program to determine the feasibility of resolving salmon from macrophytes using acoustic parameters other than target direction-of-movement or mean TS. Individual targets were visually identified as either salmon or macrophytes based on concurrent video records. Distributions of available acoustic parameters were statistically compared between the two target-types using a two-tailed t-test assuming equal variance (P = 0.05). Available acoustic parameters included echo position, pulse width (at –6, –12, and –18 dB power points), amplitude, beam pattern factor and target strength. Pulse width standard deviation (PWSD) measurements were determined to be the most effective individual parameters for discriminating macrophytes from salmon. Based on PWSD at the –6 dB echo power points, 77% of all macrophyte targets were removed from the mixed data set. Multiple selection criteria increased total target discrimination. Applying a combination of –6 and –12 dB PWSD, and Y–axis target slope criteria rejected 94% of all macrophytes, retaining all salmon targets.     

Mosaic environments shape the distribution of Neotropical freshwater ichthyoplankton

Dam presence results in heterogeneous aquatic environments that probably favour segregation of ichthyoplankton developmental stages. Therefore, the present study tested the hypothesis that dams in the Upper Uruguay River create heterogeneous aquatic environments favouring segregation of ichthyoplankton developmental stages and also separate species with different reproductive tactics. Ichthyoplankton were captured bimonthly during 2002–2015 in 25 sampling stations grouped by fluviometric characteristics. Sampling was performed at night, with cylindrical–conical plankton nets and light traps. After quantifying ichthyoplankton, we identified larvae to the lowest possible taxonomic level and classified them based on developmental stage and reproductive tactics. Egg abundance was higher in lotic environments, preflexion individuals were more abundant in transitional lotic/lentic environments, and the flexion stage was predominant in lentic environments. Larval assemblages consisted mainly of sedentary species or those with short reproductive migrations. Species with similar reproductive tactics occurred in different environments. In conclusion, we provided evidence demonstrating that mosaic environments in the Upper Uruguay River are important for life cycle completion of different fish species. These distinct habitat zones allow for successful spawning and offspring development, thus guaranteeing recruitment.     

A comparison of methods for calculating Catch Per Unit Effort (CPUE) of gill net catches in lakes

Fish constitute an important component of lake ecosystems and many different methods have been used for fish assessment. Based on gill net catches in two stratified (max depth = 14–22 m) eutrophic Danish lakes, relative fish abundance measured as Catch Per Unit Effort (CPUE) was calculated. We used three different methods of which two followed the European standard based on benthic nets (CEN (European Committee for Standardization), 2005: EN 14757. Water quality – Sampling of fish with multi-mesh gill nets. Brussels, 27 pp.), one assuming equal volumes in all depth strata and the other using calculated volumes in the depth strata. The third method followed a modified CEN standard, adopted as a new Danish (DK) standardized method based on calculated benthic and pelagic water volumes and by including both benthic nets and, compared to the CEN standard, an increased fishing effort with pelagic nets. Fish were concentrated in the littoral/benthic part of the upper two depth strata (0–6 m depth) with an up to 8 fold higher abundance than in the pelagic. Calculated CPUE is highly sensitive to the morphometry of the lakes. In lakes with extreme morphometry (unequal volumes in depth strata) it is important to use calculated water volumes for the depth strata. By including information derived from the pelagic nets, total lake CPUEs were 42–56% lower than CPUE values based on benthic nets only. We further show that the relative contribution of CPUE between habitats changes markedly with the nutrient level in 12 deep lakes. It is concluded that for deep lakes it is of key importance to include pelagic nets when comparing fish assemblages and abundances among lakes and when evaluating effects of major changes in key environmental factors, such as nutrient loading and climate.     

Comparative analysis of ichthyoplankton sampling gear for early life stages of walleye pollock (Theragra chalcogramma)

Comparative gear tows were conducted during the summers of 1987–1990 in the Gulf of Alaska. Efficiencies of four types of ichthyoplankton sampling gear were compared with respect to total catch and size distribution of larval fish. Fish were sampled using the bongo net (333 and 505 μm mesh), 1 m Tucker net, 3 m Tucker net and the Methot frame net. Results indicate that the bongo and the 1 m Tucker nets caught similar numbers and size distribution of fish. The larger 3 m Tucker and Method frame nets caught a larger size class of fish but a smaller total catch. Although there was a slightly significant diel difference in total catch detected in the smaller nets, no diel differences in size distribution of fish caught were detected for any of the nets.     

Hydroacoustic separation of rainbow smelt (Osmerus mordax) age groups in Lake Champlain

Separate assessment of young-of-year (YOY) and yearling-and-older (YAO) fish is desirable from both ecological and management perspectives. Acoustic assessments provide information on fish population size structure in the target strength (TS) distribution, but interpretation of TS distributions must be done carefully, as single age groups can produce multiple TS modes. We assessed the ability of in situ TS distributions to identify Lake Champlain rainbow smelt (Osmerus mordax) age groups in June, July, and September of 2001 using mobile and stationary surveys, knowledge of vertical distribution preferences, and predicted TS from trawl catches. YAO rainbow smelt (93–179 mm total length) had wide TS distributions between −60 and −35 dB in all 3 months with two modes at approximately −50 and −40 dB. Most stationary survey single-fish tracks attributed to YAO had targets in both TS modes and a wide TS range often over 15 dB. Between June and September, YOY rainbow smelt TS increased, but single-fish tracks were unimodal, and the TS range was smaller (6 dB). Overlap in TS attributed to YOY and YAO increased from no overlap in June (YOY TS −76 to −61 dB, 15–25 mm) to moderate overlap in July (−76 to −50 dB, 25–63 mm) to considerable overlap in September (−68 to −45 dB, 33–80 mm). In June and July, the TS distribution changed abruptly at the thermocline, indicating almost complete separation of the two groups. A more gradual TS transition was evident in September, indicating substantial overlap between YOY and YAO. Separate estimates can be obtained in September by decomposing TS overlap into components attributed to YOY and YAO rainbow smelt. However, this decomposition introduces additional uncertainty and an assessment in July or possibly August is preferable to obtain separate abundance estimates of YOY and YAO.     

Typology and behaviour of tuna aggregations around fish aggregating devices from acoustic surveys in French Polynesia

Eighty-seven two-hour acoustic surveys (radius 0.8 nautical mile, vertical range 0–500 m) around 17 fish aggregating devices (FADs) were conducted in French Polynesia between December 1995 and February 1997. Associated tuna densities were calculated using two different techniques: echo counting when the fish had sufficient distances from each other and echo integration when the fish swam close together (in schools). No acoustic detection of tuna was observed during 27 of the 87 surveys, representing 81 % of all the nocturnal surveys and 15 % of the diurnal ones. The 60 other surveys showed three different classes of aggregations: (1) ‘deep scattered fish’, observed 45 times, (2) ‘intermediate scattered fish’, observed 16 times, and (3) ‘shallow schooling fish’, observed 16 times. Sometimes aggregations of different classes were observed beneath the same FAD. The size of the fish inside the aggregations (determined from target strength values), the distance between the individuals, and the depth of the fish all decreased from ‘deep scattered fish’ to ‘shallow schooling fish’ (100–300 m for ‘deep scattered fish’, 50–150 m for ‘intermediate scattered fish’, and above the depth of 50 m for ‘shallow schooling fish’). Fish densities also varied according to the class of aggregations: 7.3, 26, and 801 fish per km³ on average for ‘deep scattered fish’, ‘intermediate scattered fish’, and ‘shallow schooling fish’, respectively. The highest densities were observed during daytime, while night-time observations indicated a variety of situations, from the absence of individuals to large amounts of fish.     

Spawning run of Atlantic Salmon (Salmo salar) in the River Tornionjoki monitored by horizontal split-beam echosounding

Fixed location split-beam horizontal echosounding was used to assess the size and timing of the Atlantic salmon (Salmo salar) spawning run in the River Tornionjoki. Four transducers, two on each river bank, were mounted across the river at the study site 4 km upstream from the river mouth. Net weirs were used on both shores to direct the passage of fish through the acoustic beams. Hydroacoustic monitoring covered 40–50% of the river cross-sectional area. Also test fishing and yearly catch statistics of salmon were used as an indication of the size of the spawning run in the river. Altogether, 7 700, 5 300 and 4 300 salmon-sized targets (target strength, TS ≥ –29 dB) moving upstream were detected in 1997, 1998, and 1999, respectively. The fish migration began in all the years by early June and peaked during the second half of the month; the migration period of large salmon lasted until mid-July. The observations made by the echosounding and catch statistics were similar in this respect. In 1998 and 1999, however, more targets of TS ≥ –29 dB were detected during late summer than could be expected by the river catches of salmon. It may be that the large targets in late summer were, in fact, whitefish whose run occurred during the same time. Hydroacoustic estimation of the total salmon run at the study site was found difficult. The numbers of salmon-sized targets detected were almost the same as the numbers of salmon caught each year by fishermen. Therefore, only an index of the run timing and the size of the stock can be produced from the data. It was clear that a considerable amount of fish escaped the acoustic monitoring by using areas uncovered by the beam, such as gaps in the bottom and the surface layers of the water column near the shores. Moreover, it was found that species recognition based on TS only is not adequate in multispecies environments. Assessment of spawning runs remains, however, a key issue in the management of Baltic salmon, and with further development, the hydroacoustic monitoring may be the most viable means of doing it.     

Biofouling development in marine fish farm influenced by net colour,immersion period and environmental conditions

Biofouling poses low productivity and high maintenance in aquaculture. This study simulated the nets of fish cages to investigate biofouling composition and development as influenced by net colour, immersion period and environmental conditions. Yellow and green net panels were immersed for 60 days and sampled at 7, 15, 30, 45 and 60 days. Results revealed that biofouling composition was dominated by Bryozoan Bugula sp. (31%–38% cover), tanaids Tanaidae (64–146 individuals.100 cm^‐2) and nematodes Linhomoeidae and Oncholaimidae (142–189 individuals.100 cm^‐2). Biofouling development was observed in succession patterns of harpacticoid–nematode–amphipod–tanaid in yellow nets and amphipod–nematode in green nets. Water velocity within the cages significantly and positively influenced the total cover, biomass and the densities of amphipods, bivalves, polychaetes, fish eggs and bryozoans. Polychaetes, gastropods, fish eggs and macroalgae were present in green nets and absent in yellow nets at 7 days. Uniformity of biofouling assemblage in both net colours was observed beyond 7 days which may be largely explained by brown biofilm formation. The result of the study revealed the dynamics of biofouling in a marine fish farm and provides vital information for developing strategies in biofouling management to lessen its impacts.     

Influence of diurnal behaviour rhythms and water-level fluctuations on the migratory activities of fish in a backwater of the River Danube:a hydroacoustic study

This paper reports the results from three fixed-location acoustic surveys carried out in August and September 1996 to analyse the influence of diurnal behaviour rhythms and water-level fluctuations on the migratory activities of fish in the mouth of the Regelsbrunn-arm east of Vienna, Austria. A BioSonics 105 dual-beam echosounder (420 kHz), with a circular transducer (6°/15°) beaming nearly horizontal across the arm was used, with acoustic ranges of between 20 and 30 m. All surveys were conducted during day and night. The fish density was highest during the first observation period with low water level, lowest during the second sampling period with increasing water level, and intermediate during the third period with decreasing water level. The fish density was always higher during night than during day. Diurnal behaviour rhythms influenced the fish density as well as the size-frequency distribution. Increasing or decreasing water-gauge superposed the influence of diurnal behaviour rhythms on the size-frequency distribution for immigrating fish of certain length classes. Fish within the length group 10–15 cm tended to migrate upstream during decreasing water-gauge. Fish above 400 mm total length tended to move upstream during increasing water-gauge.     

Intra‐annual patterns of coastal larval fish assemblages along environmental gradients in the northeastern Mediterranean

To date, little is known about the larval‐fish assemblages (LFAs) in the eastern Mediterranean. The purpose of this study is to investigate the intra‐annual variations of LFAs with an emphasis on the effects of physical, chemical, biological and meteorological conditions in the gulf of Iskenderun, which is a shallow (100 m maximum depth), semi‐enclosed bay located in the northeastern corner of the Mediterranean. The gulf is under the influence of offshore waters throughout the year. The study was based on an ichthyoplankton survey program performed at 28 stations over 12 months, between November 2009 and October 2010. During the study, a total of 11,411 larval individuals belonging to 177 taxa were sampled. The jackknife estimate of maximum species richness was 225 ± 19 for the study area. The maximum larval richness (84 species) and abundance (827 larvae/10 m²) values coincided with spring bloom in April. Gobiids dominated the ichthyoplankton of Iskenderun Bay by constituting 9% of species richness and 42% of total individuals. Four LFAs were identified which were in early winter (November to January), late winter (February and March), spring (April to June) and summer (July to October). The intra‐annual variations of richness, abundance and composition of ichthyoplankton were significantly correlated with the temperature and mesozooplankton biomass. The wind conditions and thermal stratification were also significantly correlated with the composition of LFAs. In conclusion, phenology of LFAs were formed under the influence of physical and tropho‐dynamic conditions in Iskenderun Bay, Northeastern Mediterranean.     

Vertical behavior of juvenile yellowfin tuna Thunnus albacares in the southwestern part of Japan based on archival tagging

The behavior of juvenile yellowfin tuna Thunnus albacares in southwestern Japan was investigated using archival tag data from five fish (fork length 52.5–92 cm, days at liberty 26–280 days) released near the Nansei Islands (24–29°N, 122–130°E). Vertical behavior was classified into three patterns: “shallow” (≥50 % of daytime hours at depth of <50 m), “deep” (≥50 % of daytime hours at ≥100 m), and “intermediate” (other than “shallow” or “deep”). The pooled proportion of the number of days of each behavior was 29, 25 and 46 %, respectively. The proportion of “shallow” behavior increased with fish size. The proportion of time spent near the surface at nighttime increased in the colder season, when the thermal gradient was relatively small. Surface-oriented behavior (fish remained at a depth of <10 m for more than 10 min) occurred mainly during nighttime and between November and January. Dives exceeding 500 m were occasionally observed (0.02 day^?1), and one fish dived to 1230 m. The results of our study show that yellowfin tuna were typically distributed in the mixed layer or upper thermocline where the water temperature was close to the sea surface temperature and that the vertical behavior was variable.     

Movements and behaviors of swordfish Xiphias gladius in the United States Pacific Leatherback Conservation Area

This study reports on the movements of swordfish tagged within the Pacific Leatherback Conservation Area (PLCA), an expansive region (>500,000 km²) off the U.S. West Coast that has been seasonally restricted to drift‐gillnet fishing since 2001 to reduce leatherback sea turtle (Dermochelys coricea) interactions. Thirteen swordfish were outfitted with satellite‐linked archival tags scheduled for short (2–20 days, n = 11) and longer‐term (150 days, n = 2) data collection. All tags were deployed on basking swordfish using traditional harpoon‐based methods during the fall of 2012–2013, near offshore seamounts (35.6°N/122.9°W to 37.4°N/123.5°W). Depth and temperature data from 11 swordfish (~90 to 150 kg) resulted in <251 days of movement information from the PLCA region. All tagged individuals exhibited surface‐oriented nocturnal movements, spending >99% of the night above the average thermocline depth (37.5 m), with an average night depth of 8.3 ± 1.6 m. Daytime depth distribution was greater and more variable (mean 107.1 ± 21.2 m), with fish primarily displaying three behavioral patterns: (i) basking activity, 16.7% of the day, (ii) a mixed‐layer distribution between 3 m and the thermocline (26.8% of the day), and (iii) prolonged dives below the thermocline, 56.5% of the day. For seven of the tracks, daytime basking rates increased when thermocline depth was <37 m. As fish moved offshore, there was less variability in vertical movements with a reduction in both basking activity and mixed layer occupancy, as well as an increase in average daytime depth. These data are discussed with respect to the potential development of alternative fishery options for the PLCA.     

Pinger affects fish catch efficiency and damage to bottom gill nets related to bottlenose dolphins

There is some evidence that the presence of Tursiops truncatus in fishing areas represents a real economic threat to fishermen due the dolphin feeding on the entangled fish, damaging the nets and reducing the fish catch. We have carried out experiments to assess the efficiency of a pinger in decreasing the interaction between the dolphins and fishing nets, in a fishing area off the coast of southern Italy, where Tursiops truncatus is frequently observed to interact with bottom gill nets. Two identical monofilament bottom gill nets (900 m long), one equipped with pingers and the other without, were used to measure the effect of these pingers on the abundance of the catch and net damage. For each haul (58 in total), data on dolphin sightings near the nets, damage judged to have been done by dolphins, weight and species composition of the catch were collected. All damage to the nets were recorded at the end of each haul. Dolphins in the fishing area were sighted 11 times out of 29 fishing activities (38%). The net equipped with pingers contained 28% more fish (biomass) than the net without pingers (t test, P < 0.04) and was less damaged (−31%, t test, P < 0.01). To assess whether the efficacy of these pingers remain constant over long period, long-term experiments should be carried out.     

Effects of anthropogenic noise as a source of acoustic stress in wild populations of Hippocampus guttulatus in the Ria Formosa,south Portugal

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Observations of controlled moving targets with split-beam sonar and implications for detection of migrating adult salmon in rivers

When measuring the flux of migrating salmon in rivers there are often multiple fish in the beam simultaneously. To obtain accurate measurements of flux, an understanding of the effects of multiple targets is required. Multiple targets in various configurations were passed through a horizontally-oriented 4° × 10° beam from a split-beam echo sounder. The effects on target strength, detection probability, spatial and temporal patterns of echoes, and measurement of target position are presented. When multiple targets were in the beam at the same range, the target strength was positively biased, whereas, when they were in-line at different ranges, the target strength and detection probability were negatively biased for the shadowed targets. Targets at the same range, but separated in the vertical or horizontal direction, produced characteristic patterns in the X vs. ping and Y vs. X plots. Similar patterns were found in routine observations of migrating salmon, allowing the identification of some multiple-target events. Identification of these events can aid in the correct interpretation of migrating fish data for flux measurement.     

Side-aspect target strength of Atlantic salmon (Salmo salar), brown trout (Salmo trutta), whitefish (Coregonus lavaretus), and pike (Esox lucius)

The side-aspect acoustic target strengths (TS) of 19 Atlantic salmons (Salmo salar), 16 brown trouts (Salmo trutta), 10 whitefish (Coregonus lavaretus) and 9 pikes (Esox lucius) were measured using a 200 kHz split-beam echosounder, in order to study the relationship between TS and fish size indices (length, weight and side area). The effect of side aspect angle on TS was also studied. Linear models between TS and the logarithm of the fish size indices were fitted with length being best for predicting TS. Typically, the standard error of estimate was 1.2–2.9 dB. The side-aspect TS measurements with specimens of known size showed that the linear relationship between full side-aspect TS and the logarithm of fish length for salmonid (Salmo salar + Salmo trutta) was on average 4.7 dB (SE = 0.7), lower than for whitefish and pike combined. The effect of side aspect angle on TS was modelled with cos³ (2α) function. The differences in the TS between full side aspect and head/tail aspect were 17.4, 19.0 and 19.6 dB for salmonid, whitefish, and pike, respectively.     

Density and in situ target strength of herring and sprat: a comparison between two methods of analyzing single-beam sonar data

We analyzed sonar data obtained with a 70-kHz single-beam echosounder with two methods for removing the beam pattern effect: one based on the Craig and Forbes logarithm; the other based on a z-transformation and deconvolution operation. The sonar data were collected from 4 July to 23 October 1985, in a coastal area of the northern Baltic proper where herring (Clupea harengus L.) and sprat (Sprattus sprattus L.) dominated the pelagic fish populations. The two methods gave similar total fish densities, but the proportion of different size classes sometimes differed by up to 30 percentage units.The acoustic data are compared with size distributions and catch per unit effort in simultaneously collected vertical gill-net samples. Size distributions in nets and acoustics were fairly similar, but the relationship between catch per unit effort in gill nets and acoustic density estimates was poor. Target strength to fish-length regressions from the two acoustic methods were similar even though individual estimates differed by up to 4 dB on some sampling occasions. When the slope of the regression is set to 20 we obtained the equation TS=20 log L-69.9 with both methods (TS is target strength in dB and L is fish length in cm).     

Acoustic evaluation of the vertical distribution of dwarf ayu Plecoglossus altivelis altivelis in Lake Biwa

SUMMARY: The vertical distribution of dwarf ayu Plecoglossus altivelis altivelis in the pelagic waters of Lake Biwa was evaluated from June to September in 1995–97 from eight acoustic surveys using a quantitative echosounder. In each survey, echoes from a depth range of 3 m to the sea bottom were collected at a station every 2.7 s for 24 h together with measurements of vertical profiles of water temperature and chlorophyll-a. The ayu's echoes were identified using an underwater video camera. The ayu were observed near the maximum chlorophyll-a depth and above the thermocline. Their density was highest at depths of 4–11 m with 2–4 individuals/m ³ and was almost zero below 20 m. Echo signs were recorded as having a frequent duration of more than 1 h at night, whereas were of a shorter duration in the day. The fish stay in the epilimnion during the day without any clear vertical migration, but are distributed more uniformly at night. The advantages of remaining in the epilimnion are discussed in terms of food availability and predator avoidance.     

Biofouling of salmon cage netting and the efficacy of a typical copper-based antifoulant

With the relatively recent advent of intensive aquaculture practices, there is a need for data on the contemporary biofouling and antifoulants associated with it. Therefore, experiments were conducted to yield data on the biological communities that develop on contemporary fish cage netting, both antifouled and untreated. The efficacy of a typical copper-based antifouling treatment was assessed through netting immersion trials performed at a salmon farm site in Clift Sound, Shetland. Over a 10-month period, regular samples were taken and data generated on the fouling biomass and species composition of the communities that developed. Furthermore, percentage net aperture occlusion values (PNO) were calculated with the aid of an image capture and analysis system, which is a methodology that shows excellent potential for use in future net fouling trials. Over the course of the trial, PNO values calculated from control netting rose quickly, but remained low for 150 days longer on treated nets; by day 210, PNO values were 98.72% and 3.06% for control and copper nets, respectively, immersed at 3 m. Following 10 months immersion, treated nets significantly reduced the biomass accumulation of biofouling compared to control nets (1.8 kg m^− 2 as opposed to 4.9 kg m^− 2). A total of 40 separate taxa were identified during the study, and species richness and composition data were significantly affected by the application of copper-based antifouling; for example, average species richness values were 16 and 9.75 for control and copper nets, respectively, immersed at 1 m. Of significance, the species Mytilus edulis and members of the Ascidiacea, including Ciona intestinalis and Ascidiella aspersa, were observed fouling the copper-treated netting.     

What goes up must come down: Diel vertical migration in the deep‐water sablefish (Anoplopoma fimbria) revealed by pop‐up satellite archival tags

The sablefish (Anoplopoma fimbria) is a long‐lived species with wide distribution throughout the North Pacific Ocean. While adult sablefish are considered a deep‐water fish, diet analyses suggest that they undergo vertical migrations that could be related to prey movement and feeding. Pop‐up satellite archival tags (PSATs) were used to observe the fine‐scale depth selection behavior of adult sablefish tagged off the Washington coast during the summer from June to August. Tags were physically retrieved after they surfaced using direction‐finding equipment so that complete datasets over the entire deployment were obtained from 14 tags. PSATs that recorded depth and temperature every 4 min during the deployment confirm that sablefish inhabit depths of 750 m or greater. However, a majority of the tagged fish underwent extensive vertical migrations that averaged 254.4 m overall and occurred at a 24 hr periodicity. Variations were observed among individuals in the amount of the deployment during which vertical migrations occurred, ranging from 12.37% to 63.48% of the time. During the vertical migration, fish ascended towards the surface at night and descended prior to daylight (i.e., diel vertical migration). Sablefish generally inhabited temperatures of 5°C but during the vertical migrations were found at temperatures from 6 to 10°C. Sablefish are opportunistic feeders with a large proportion of their diet being fish, euphausiids and cephalopods. Because these prey items also exhibit diel vertical migrations, it is possible that the vertical migratory behavior displayed by the sablefish was in response to the movements or the location of their prey.