Seasonal abundance of two important forage species in the North Pacific Ocean,Pacific herring and walleye pollock

Pacific herring (Clupea pallasii) and walleye pollock (Theragra chalcogramma) are important forage species in the North Pacific Ocean, but their seasonal abundance patterns are poorly known. During three consecutive years of monthly acoustic surveys in Lynn Canal, southeastern Alaska, large schools of herring dominated during winter and were present in a 60-km long submarine gully; this gully appears to provide critical winter habitat for herring when their prey are less available and energy expenditure must be reduced. The salient change in pollock distribution is a shift from shallow waters during summer to deeper waters during winter, such that shallow (<40 m) waters are nearly devoid of pollock during winter. The shift presumably occurs in response to the build-up of secondary productivity during summer and predator avoidance during winter. The seasonal changes in herring abundance drove seasonal changes in predator abundance, as Steller sea lions (Eumetopias jubatus) and humpback whales (Megaptera novaeangliae) preyed upon the winter aggregation of herring. Such seasonal links likely serve an important role in structuring trophic relationships in the North Pacific Ocean ecosystem.     

Observation and quantification of two incidents of mass fish kill of Icelandic summer spawning herring (Clupea harengus) in the winter 2012/2013

The Icelandic summer‐spawning herring (Clupea harengus) stock overwinters in large, dense schools like other herring stocks. In the winter of 2012/2013 around 300 thousand tonnes, or ~70% of the spawning stock, overwintered in a fjord west of Iceland. The inner part of the fjord, where the herring was located, is separated from the outer part with a natural barrier and a built‐up road with a 210 m long bridge. This creates strong tidal currents under the bridge. On 14 December and again on 1 February mass mortalities of herring took place in this location, and the sea floor and the shores were covered with dead herring. Fieldwork, including camera and video recordings on the shore and on a small boat, was conducted 3 and 4 days after the incidents. Results from this indicated that a total of 55 thousand tonnes had died during these two incidents, an amount nearly equal to the total catch from the stock that year. Measurements of environmental conditions in the days following the incidents showed that the oxygen saturation was generally 20%–40% but was as low as 15% (1.1 ml/L). The most likely explanation for the mortalities was oxygen depletion resulting from respiration of the large herring biomass, limited atmospheric‐water gas exchange due to calm and cold weather prior to both incidents and sea ice on part of the fjord, and limited renewal of water coming in and out via tidal currents. Aerobic decomposition of dead herring came additionally in the latter incident.     

Marine bird response to forage fish during winter in subarctic bays

Despite the importance of understanding marine bird response to prey fish, few studies have examined this relationship during winter. Over a 5‐year period, we conducted synoptic marine bird and hydroacoustic forage fish surveys during early (November) and late (March) winter to characterize the factors influencing marine bird and forage fish dynamics at two spatial scales (fish school and bay) within subarctic bays of coastal Alaska, USA. Over 40% of observed marine birds were associated with a fish school (within 150 m of a fish school), although only 20% of fish schools were associated with birds. Seasonally, we recorded significantly more schools during early winter. The marine bird community also shifted from being comprised primarily of marbled murrelets (Brachyramphus marmoratus) and large gulls (Larus spp.) in early winter to common murres (Uria aalge) in late winter. At the school level, marine birds were more likely to be associated with shallow fish schools within 500 m of shore and in smaller prey patches. At the bay level, gull abundance was positively associated with the total number of fish schools recorded, while diving birds were more abundant when fish schools were higher in the water column, in shallower bottom depths, and in areas with more eel grass habitat. Our results indicate the importance of temporal, habitat, and fish school variables as drivers of marine bird presence and abundance, underscoring the complexity of predator–prey dynamics in the marine environment during winter.     

Factors driving spatial variation in egg survival of an ecologically and culturally important forage fish

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Impact of environmental deviations on the larval and year-class abundances in the spring spawning herring (Clupea harengus membras L.) of the Gulf of Riga (Baltic Sea) in 1947–2004

Regular weekly sampling of larvae with a Hensen net in the stationary stations on the main spawning grounds of the brackishwater herring population in the Gulf of Riga was carried out in 1947–2004. Probably due to the rapidly increasing anthropogenic eutrophication and the rising winter temperature the yearly abundance of herring larvae has significantly increased with a shift in the average abundance in 1972/1973. Correlation of the average number of herring larvae with the year-class strength was significant for the intermediate (10–16 mm, P < 0.05) and large (≥17 mm, P < 0.001) larval size groups. The monthly average air temperature in the period from January to March was the main environmental variable regulating the abundance of the year classes (P < 0.001). The dependence between the spawning stock biomass (SSB) and year-class abundance differed by climate periods. In the period of cold winters environmental conditions dominated in determination of the year-class abundance whereas the role of the SSB was nonsignificant (P > 0.05). In the period of milder winters the importance of the SSB in determination of the abundance of year classes considerably increased and it significantly correlated with the abundance of year classes (P < 0.05). The winter–spring temperatures and other climatic variables presumably control the abundance of the forming year-class during the period of the highest larval mortality mainly via the production of their food.     

Pacific herring (Clupea pallasii) consumption by marine birds during winter in Prince William Sound,Alaska

Following the 1989 MV Exxon Valdez oil spill (EVOS) and subsequent herring population collapse in Alaska's Prince William Sound (PWS), the Pacific herring (Clupea pallasii) fishery was closed. In the 25 yr since EVOS, herring and several herring‐dependent marine bird species have failed to reach pre‐spill population levels. One hypothesis is that intense predation pressure may be inhibiting herring recovery. To inform herring modeling efforts, this study estimated marine bird consumption of juvenile and adult herring in PWS for 10 winters over an 18‐yr period (1989–90 through 2006–2007). Total estimated herring consumption by wintering marine birds averaged 2409 ± 950 t, indicating that avian consumption represents a substantial and inter‐annually variable source of herring mortality. Common murre (Uria aalge) consumed the greatest portion (16–80%) of herring in all years among marine bird species. Juvenile herring biomass consumed annually by common murre was greater than murre consumption of adult herring biomass. Time lag analyses showed that marine bird consumption of adult herring is negatively correlated with the amount of herring spawn observed in subsequent years, but such effects were not observed more than 2 yr. Our models indicate that during years in which herring recruitment is low or bird populations are particularly large, marine birds can consume up to 10% of the annual adult herring biomass. Our results highlight the importance of herring to wintering PWS birds. We propose that future management of herring stocks seeks to reduce negative impacts on marine birds that prey on herring.     

Long‐term variation in the abundance of Pacific herring (Clupea pallasii) from the Yellow Sea in the western North Pacific and its relation to climate over the past 590 years

A qualitative understanding of the long‐term variation in the population dynamics of Yellow Sea (YS) herring is particularly important for clarifying the evolutionary processes and driving mechanisms of the YS large marine ecosystem. Unfortunately, because of a lack of long‐term, continuous, and simultaneous monitoring data, the specific driving processes and mechanisms of climate effects on the population dynamics of YS herring remain largely unknown. In response to this scientific issue, we preliminarily propose the idea of reconstructing long‐term changes in YS herring abundance over the past 590 years (AD 1417–2004) based on historical documents and attempt to explore the impacts of climate on the population. Our results show that YS herring abundance maintained at a relatively high level from AD 1417 to 1870 (during the Little Ice Age); in contrast, the population declined significantly from AD 1870 to 2004 at different rates. In addition, we also found that there were strong relationships between the population abundance of YS herring and the Pacific decadal oscillation (PDO) and drought/flood cycles. We suggest that the fluctuations in YS herring abundance may be influenced by ocean–climatic circulation shifts throughout the North Pacific, especially the PDO.     

An individual based model of fish school reactions: predicting antipredator behaviour as observed in nature

An object-orientated, two-dimensional, cellular automata (CA) model is developed to describe and predict the schooling behaviour of fish in general, with Norwegian spring-spawning herring, Clupea harengus L., being used as a case study. The CA model is applied to visualize internal school dynamics based on individual decision rules. Several antipredator strategies, such as split , join and vacuole, performed by schools during predator attack, are visualized in the model. The primary driving force of individual fish is based on simple attraction rules. The model includes stochastic elements which assume that individual herring do not have perfect information about their surroundings. Isolation of individual fish from a school during predator attack is also predicted by the model. The disruption of highly organized fish schools, followed by an attack on solitary herring individuals, may be an important tactic for predators feeding on schooling prey. The conceptual CA model identifies patterns and mechanisms both within and between schools that may be important in all schooling fish. Model simulations are compared with observed predator–prey interactions between killer whales, Orcinus orca L., and herring in northern Norway.     

A conditional simulation of acoustic survey data: advantages and potential pitfalls

Standard geostatistical techniques provide effective methods for estimating the global abundance and precision of a variable of interest, for mapping its spatial distribution and for describing its spatial structure. In the case of acoustic survey data, however, obtaining a measure of precision of the global abundance estimate is confounded by the combination of variances from the interpolation of both the acoustic data and the concomitant fish length data. Even if the global estimation variance could be calculated, the distribution of the estimation error is not known and so confidence intervals cannot be determined. Furthermore, kriged distribution maps, in minimising the estimation variance, tend to smooth out local details of the attribute’s spatial variation: small values can be overestimated and larger ones underestimated, such that the kriged map is smoother than reality. This can lead to serious shortcomings when trying to detect patterns of extreme attribute values, such as the high densities encountered in some fish schools. Stochastic geostatistical simulations, conditional on sampled locations, provide a solution to many of these problems. They can deliver a measure of uncertainty for local (density) estimates, a confidence interval estimation for the global mean density, and finally, reproduce global statistics, such as the sample histogram and variogram. In so doing, they also provide maps of the attribute, which are spatially realistic because the variogram is reproduced; these are generated as a number of equiprobable realisations. In the present paper, we apply these techniques to acoustic data from an acoustic survey of North Sea herring. Sequential gaussian simulations are used to generate realisations for fish length and values of the nautical area scattering coefficient. These are then combined to produce realisations of herring density. The combined set of multiple realisations is then used to provide confidence intervals for the global abundance estimate: 95% of the herring abundance estimates are between 5677 and 6271 millions of individuals. Although the method presented in this paper contributes to the assessment of total uncertainty for acoustic surveys, the approach may have suffered from bias due to the use of off-the-shelf data transformation algorithms on fisheries acoustic data, which are often very positively skewed. We discuss this limitation and propose corrections for future work.     

Spawning of herring: day or night,today or tomorrow?

Diel variations in schooling patterns and spatial dynamics during spawning were studied in Norwegian spring-spawning herring (Clupea harengus) off south-western Norway by acoustic surveying, diel cycle experiments and school tracking by sonar, and bottom gillnet sampling. Herring formed horizontally extensive, loosely packed demersal layers shortly after darkness. At night, the fish disappeared in the acoustic dead zone, but lifted off the bottom early in the following mornings. At daytime the herring reorganised into dense pelagic schools. The evening descent to the spawning habitat was considered as part of a precautionary strategy towards visual predators, as the bottom is a high-risk zone for archetypal pelagic fish like herring. Large numbers of gadoids, which are potential herring predators, were present in the area. Herring not ready to spawn dominated the bottom samples in 4 out of 5 days, suggesting that pre-spawning herring followed the descent of ripe herring. The herring spawning layers shifted in a south-easterly direction from day to day in diel spawning waves.     

An investigation of the biological basis of recruitment,growth and adult survival rate variability of Pacific herring (Clupea pallasi) from British Columbia: a synthesis

I explored the biological basis of variation in recruitment (age 3 abundance), growth and age‐specific adult survival rate for the major populations [West Coast Vancouver Island (WCVI), Strait of Georgia, Central Coast, North Coast and Haida Gwaii] of Pacific herring (Clupea pallasi) that inhabit British Columbian waters. The analyses were based on a synthesis of time series of empirical observations of herring population characteristics (egg deposition, age‐specific abundance and size) and prey, competitor and predator biomass/abundance. Recruitment was not correlated among populations. Recruitment variability was explained for WCVI herring only, as a consequence of prey (the euphausiid Thysanoessa spinifera) biomass during August in each of the first 3 years of life, and the biomass of piscivorous Pacific hake (Merluccius productus) during the first year of life. Recruit mass and adult mass‐at‐age were correlated among populations and over ages within populations. Recruit mass was affected by T. spinifera biomass in August of the first and third years of life. Adult mass‐at‐age variability was determined mainly by size at the beginning of the growth season, but also by T. spinifera biomass in August. Age‐specific adult survival rates were not correlated among the five populations. Survival rates decreased with age; there were additional population‐specific effects of somatic mass and T. spinifera biomass in August. The analyses were repeated using physical oceanographic explanatory variables. Only recruit mass variation was explained significantly by physical oceanographic variables, and the biological‐based explanation of recruit mass variability accounted for more of the variation.     

Distribution, density and abundance of the western Baltic herring (Clupea harengus) in the Sound (ICES Subdivision 23) in relation to hydrographical features

Biomass and duration of the over-wintering period of the Rügen spring spawning herring stock (RHS) in the Sound (ICES Subdivison 23) were investigated as well as possible hydrographical factors affecting relative distribution and triggering southwards migration towards the spawning grounds. Monitoring was performed during 27 surveys over a 6-year period (1993–1998). Abundance of 45–165 000 t in August–February, 5–60 000 t in March–May, and <2000 t in June–July was found. This indicates a longer over-wintering period than hitherto assumed. The year classes 1988, 1991 and 1994 were relatively strong occurring in the Sound from age 2 (winter-rings) and abundance in autumn 1993 and 1996–1997 was relatively high. Relative year class strengths are in accordance with variations in larval indices at the spawning grounds. Decreasing abundance in late spring in the Sound was concordant with observed peaks in commercial landings and (subsequent) peak larval indices at the spawning grounds. Larger size groups seem to migrate southwards in spring before smaller herring size groups. Peak herring densities occur in the northern Sound in autumn and they do not concentrate near the southern Drogden threshold in spring before southwards spawning migration. Highest densities were found from 8 to 22 m below sea surface, and just below or in the halocline in areas with stratified water column. Indications of inflows to the Baltic triggering southwards migration can be observed at some occasions, however, these results are not conclusive because of relatively few biomass estimates in 1996–1998. The identified patterns in herring occurrence agree with some conclusions on migration routes for RHS from previously reported tagging studies, but the present investigations indicate prolonged over-wintering in the Sound. This report quantifies the abundance and distribution of herring in the Sound over the year on basis of repeated investigations, and investigates possible influence of hydrographical factors on distribution and migration.     

Measuring copepod abundance in deep‐water winter habitats in the NE Norwegian Sea: intercomparison of results from laser optical plankton counter and multinet

The main purpose of this study was to assess the usefulness of the laser optical plankton counter (LOPC) as a Calanus finmarchicus monitoring tool in the NE Norwegian Sea in winter. To test this, a multinet and an LOPC were used simultaneously to sample overwintering copepods in the Lofoten basin in January 2007. Additional data from an LOPC laboratory experiment were also analyzed to help the interpretation of the field data. Both the laboratory data and the field data indicated that the presence of particles other than zooplankton generally contaminated the zooplankton signal in the LOPC data. However, reliable abundance estimates could be made from the LOPC data by choosing an appropriate size range in the LOPC size distribution. This size range was determined by comparing LOPC abundance estimates from systematically varying size ranges to multinet zooplankton abundance data. The average difference was smallest for a size range of about 900–1500 μm. As C. finmarchicus, stages IV and V, was by far the most abundant species in the NE Norwegian Sea, abundance estimates from the LOPC in this size range were in practice estimates of C. finmarchicus abundance, making the LOPC suitable as a tool for providing high resolution winter data in the NE Norwegian Sea.     

Fish assemblages associated with artificial reefs of concrete aggregates or quarry stone offshore Miami Beach,Florida, USA

Few studies have compared the suitability of different artificial reef construction materials in terms of their efficacy in acquiring diverse faunal assemblages. We compared the fishes associated with 12 co-located reefs constructed of limestone quarry boulders, concrete-gravel aggregate, or concrete-tire aggregate (four of each substrate) in 7 m of water, 200 m offshore Miami Beach, Florida, USA. All 12 reefs were deployed 100 m apart the same day in two lines of six. The four quarry stone reefs consist of a pile of 50 boulders each. The remaining eight reefs, of concrete-gravel aggregate and concrete-tire aggregate, were each constructed with 25 1.5 m edge and 25 1.2 m edge tetrahedron modules. Every two months from October 1998 to February 2001, SCUBA divers recorded fish species, abundance, and length, as well as spiny lobster, Panulirus argus, abundance. One hundred and forty-six species of fishes were recorded during the study period. The abundance and species richness of fish on each treatment exhibited a significant (p<0.05) seasonal variation with summer months having the greatest numbers and winter the lowest. There was no significant difference in total fish or spiny lobster abundance or fish biomass amongst the three reef types (p>0.05). Likewise, multi-dimensional scaling of Bray-Curtis dissimilarity indices did not indicate clustering of fish assemblages by reef type. Comparison of pre-deployment fish counts from the reef sites and neighboring hard bottom and jetty with counts from the same sites two years post-deployment indicate the artificial reefs increased both fish abundance and richness in the local area.     

Vortex flow produced by schooling behavior of arabesque greenling <Emphasis Type="Italic">Pleurogrammus azonus</Emphasis>

Arabesque greenling Pleurogrammus azonus form schools of 30,000–60,000 individuals to feed on mesozooplankton near the sea surface. The fish need to be close to the surface where mesozooplankton production occurs, but the food is not sufficiently dense there for the fish to grow optimally while avoiding predation by sea birds. Arabesque greenling use a unique method to optimize their feeding conditions while avoiding staying on the sea surface. When the school swims upward, water is pushed downward. Upward swimming of a school of 30,000 individuals generates a downward stream of about 0.8–1.1 m/s, resulting in a convergent flow near the surface. This convergence concentrates the mesozooplankton on the sea surface and transports them into deeper layers with a strong vortex about 3.0 m in diameter and 10–20 m long. Thus, schooling of this fish induces vortex flows that provide a rich feeding environment.     

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).     

Modelling the dispersal of herring and hake larvae in the Strait of Georgia for the period 2007–2009

The Strait of Georgia (SoG), between Vancouver Island and mainland British Columbia, is a larval rearing ground for both hake and herring stocks, which are commercially important. Year‐to‐year variability in larval retention within the strait is examined by simulating drift tracks of larvae for these species using an ocean circulation model and a particle‐tracking model. Larvae with different vertical swimming behaviors were tracked in the springs of 2007, 2008, and 2009. Since herring larvae mostly stay near the surface, their distribution is heavily influenced by the wind. Strong winds to the north soon after the hatching period tend to wash herring larvae out of SoG and winds to the south help retain herring larvae inside the Strait. In 2007, the model indicates a massive wind‐driven export of herring larvae which may have led to the observed failure of herring production. In contrast, hake larvae reside deeper in the water column (50–200 m). Their distribution is less sensitive to surface forcing but is shaped by a deep gyre with cross‐strait currents. This study also suggests that the northern and southern SoG are weakly connected for herring larvae dispersal, which makes both regions potentially important to recruitment.     

Environmental variability controls recruitment but with different drivers among spawning components in Gulf of St. Lawrence herring stocks

The factors affecting herring recruitment are still poorly understood, complicating the prediction of stock dynamics and the choice of operational management strategies. We investigated effects of intrinsic (SSB) and extrinsic factors (physical and biological environments, including competition and predation) on recruitment of the spring and fall spawning components of each of the two herring stocks occurring in the Gulf of the St. Lawrence between 1971 and 2014. Effects of potential explanatory factors on recruit (age 2) abundance were tested using Generalized Additive Models. Model fit was significantly improved by incorporating both physical and biological environmental variability, but effects of herring SSB and predation were not significant. Indices of zooplankton abundance and phenology explained more variance in recruitment than physical indices. Our results emphasize the dominance of bottom‐up processes over SSB in the regulation of herring recruitment. Environmental variability did not seem to act uniformly on the recruitment of either stock or their respective spawning components. A long‐term trend of decreasing recruitment in spring spawners was associated with a long‐term decline in abundance of cold water copepods. In fall spawners, optimal recruitment was dependent on warmer environmental conditions combined with an adequate supply (species composition and phenology) of zooplankton. These results provide the first empirical evidence that spring and fall spawning herring are adapted to contrasting environmental conditions and shed light on the potential mechanisms linking herring recruitment to key zooplankton community characteristics and phenology. Management strategies can be improved by incorporating this new knowledge on environmental drivers of herring recruitment.     

Evaluation of plankton surface pushnets and oblique tows for comparing the catch of diadromous larval fish

A bow-mounted surface pushnet and an obliquely towed plankton net were compared to evaluate gear efficiency and effectiveness in collecting larval fishes under daytime and nighttime conditions. The diadromous species targeted were striped bass Morone saxatilis, white perch, Morone americana, and river herring Alosa sp. We sampled the lower Roanoke River, North Carolina, from March through June of 2002 and 2003. Striped bass, white perch and river herring represented over 90% of the larvae collected during the study period. Mean larval densities (number/100 m³) were 63.4 for striped bass, 26.4 for river herring, and 17.7 for white perch. Striped bass larval densities were significantly higher in the surface pushnet for both years (P ≤ 0.05). In 2002, white perch mean larval density was significantly higher at night in the surface pushnet samples, but in 2003 there were no differences between day and night samples. River herring mean densities were significantly higher in the surface pushnets for both years, but showed no clear patterns between day and night samples. Larger larvae were consistently collected in the surface pushnets for all species. Overall, the surface pushnet was easier to operate. The pushnet was mounted on the bow of a small jon boat and required less specialized gear and fewer personnel than oblique sampling. The method also allows for sampling in shallow water or vegetated habitats. Because larvae were significantly larger in the surface samples, using surface pushnets may not allow for detection of the smaller-sized larvae therefore underestimating the abundance of smaller fish. Depending on the question being asked, we recommend that sampling programs should use both gear types to reduce any gear biases.     

Potential implications of acoustic stimuli as a non‐physical barrier to silver carp and bighead carp

The effectiveness of an acoustic barrier to deter the movement of silver carp, Hypophthalmichthys molitrix (Valenciennes) and bighead carp, H. nobilis (Richardson) was evaluated. A pond (10 m × 5 m × 1.2 m) was divided in half by a concrete‐block barrier with a channel (1 m across) allowing fish access to each side. Underwater speakers were placed on each side of the barrier opening, and an outboard motor noise (broadband sound; 0.06–10 kHz) was broadcast to repel carp that approached within 1 m of the channel. Broadband sound was effective at reducing the number of successful crossings in schools of silver carp, bighead carp and a combined school. Repulsion rates were 82.5% (silver carp), 93.7% (bighead carp) and 90.5% (combined). This study demonstrates that broadband sound is effective in deterring carp and could be used as a deterrent in an integrated pest management system.