Modelling potential spawning habitat of sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus) in the Bay of Biscay

Large amplitude variations in recruitment of small pelagic fish result from interactions between a fluctuating environment and population dynamics processes such as spawning. The spatial extent and location of spawning, which is critical to the fate of eggs and larvae, can vary strongly from year to year, as a result of changing population structure and environmental conditions. Spawning habitat can be divided into ‘potential spawning habitat’, defined as habitat where the hydrographic conditions are suitable for spawning, ‘realized spawning habitat’, defined as habitat where spawning actually occurs, and ‘successful spawning habitat’, defined as habitat from where successful recruitment has resulted. Using biological data collected during the period 2000–2004, as well as hydrographic data, we investigate the role of environmental parameters in controlling the potential spawning habitat of anchovy and sardine in the Bay of Biscay. Anchovy potential spawning habitat appears to be primarily related to bottom temperature followed by surface temperature and mixed‐layer depth, whilst surface and bottom salinity appear to play a lesser role. The possible influence of hydrographic factors on the spawning habitat of sardine seems less clear than for anchovy. Modelled relationships between anchovy and sardine spawning are used to predict potential spawning habitat from hydrodynamical simulations. The results show that the seasonal patterns in spawning are well reproduced by the model, indicating that hydrographic changes may explain a large fraction of spawning spatial dynamics. Such models may prove useful in the context of forecasting potential impacts of future environmental changes on sardine and anchovy reproductive strategy in the north‐east Atlantic.     

Tests of larval retention in a tidally energetic environment reveal the complexity of the spatial structure in herring populations

Dispersion during the larval phase is of central importance in the dynamics of marine fish and invertebrate populations. Rapid transport or dispersion of larvae may contribute to connectivity and mixing, whereas spatial persistence (retention) is hypothesized to favour stock complexity and local subpopulations. Larval retention, while rarely quantified, may be defined in species with protracted spawning by the spatial co‐occurrence of larvae of different sizes or ages. The spatial distributions of larval Atlantic herring (Clupea harengus) were examined from 22 annual autumn surveys (1975–1998) and 9 spring surveys (1975–1984) from the Bay of Fundy, a region with large tides and residual flow. Larvae of all sizes (3–27 mm in length, from hatch to nearly 4 months post‐hatch) were observed each year in two major aggregations; one off southwestern Nova Scotia, and the other in the mid‐inner Bay of Fundy off the northwestern shore of Nova Scotia. Two similar aggregations were evident over 5 months later from 9 spring surveys (1975–1984), despite the residual flow that would have swept the larvae from the region within 1 month. Larval retention was apparent from overlapping centres of mass of different size (=age) classes of larvae, and tested using a size diversity index, based on the co‐occurrence of 1‐mm‐size categories, derived from protracted spawning of several weeks. Geospatial ‘hot spots’ (G_i* statistic) of four size (age) classes were evident at specific stations in the 50–100 m bathymetric zone and not elsewhere. These metrics provide quantitative measures of retention that may be applied to many ichthyoplankton data sets. One of the three main spawning areas collapsed during the study period after a period of intense fishing and failed to rebuild, but there was no substantial change in the location of larval hotspots in subsequent years. While larval retention does not directly relate to each spawning location, larval retention in the Bay of Fundy contributes to the complex ‘metapopulation’ structure of herring stocks in the western Atlantic.     

Distribution of albacore (Thunnus alalunga) in the Indian Ocean and its relation to environmental factors

The distribution pattern of albacore, Thunnus alalunga, in the Indian Ocean was analyzed based on catch data from the Taiwanese tuna longline fishery during the period 1979–85. The Taiwanese tuna fishery began operating in the Indian Ocean in 1967. We used a geographic information system to compile a fishery and environmental database and statistically explored the catch per unit effort (CPUE) distribution of albacore. Our results indicated that immature albacore were mainly distributed in areas south of 30°S although some displayed a north–south seasonal migration. Mature albacore, which were mainly concentrated between 10°S and 25°S, also showed a north–south migration. Within 10°S and 30°S, the separation of mature, spawning, and immature albacore life history stages roughly coincided with the boundaries of the three oceanic current systems in the Indian Ocean. The optimal environmental variables for CPUE prediction by stepwise discriminant analysis differed among life history stages. For immature albacore, the sea surface variables sea surface temperature (SST), chlorophyll concentration and surface salinity were significant. For mature albacore, SST was significant, while for spawning albacore, the sub‐surface variables temperature at 100 m and oxygen at 200 m were significant. Spawning albacore evidently prefer deep oceanographic conditions. Our results on the oceanographic conditions preferred by different developmental stages of albacore in the Indian Ocean were compatible with previous studies found in the Pacific Ocean.     

Spawning grounds of red sea bream in the east Seto Inland Sea

We surveyed the distribution of red sea bream eggs between 2006 and 2010 in the east Seto Inland Sea to determine the location of the spawning grounds and the characteristics of preferred spawning habitats. We identified early-development stage red sea bream eggs using a monoclonal antibody assay. The early-development stage eggs were present in waters around Awaji Island (AW area) and around Ieshima Island, Shodo Island, and Bisan Seto (collectively the ISB area). The mean red sea bream egg density in the AW area was not significantly different from that in the ISB area. Based on the locations where we observed early-development stage eggs and the density of eggs, the ISB and AW areas appeared to be the primary spawning grounds in the east Seto Inland Sea during the survey period. A quotient analysis suggested that spawning fish had clear preferences for physical conditions. The areas suitable for spawning appear to be those with sandy substrate at a depth of 30–70 m. The temperature range suitable for spawning was between 16.5 and 21.5 °C.     

Growth,sexual maturation,and spawning season of Atlantic halibut,Hippoglossus hippoglossus,in Faroese waters

Biological data on Atlantic halibut, Hippoglossus hippoglossus, caught in an exploratory bottom trawl fishery in November–February 1983–1986 in a recently discovered deep water spawning area and in other both coastal and bank areas in Faroese waters, are presented.Studies of increases in length and weight confirmed the sex-dependent growth patterns found in earlier works, i.e. a higher growth rate in females than males especially after sexual maturity. The data show that Faroese halibut grow much more rapidly and mature at lower average ages (c. 4.5 years in males and slightly above 7 years in females) than in any other area investigated in the North Atlantic. Average length and round weight at sexual maturity were slightly above 55 cm and c. 1.7 kg in males, while the corresponding values in females were 110–115 cm and c. 18 kg.Spawning in the area appeared to commence in January and intensify in February. The peak in spawning activity was apparently not attained during the study period (November–February), and most probably occurred later in winter or early in spring.     

Competitive exclusion of brown trout Salmo trutta L., by rainbow trout Oncorhynchus mykiss Walbaum, in lake tributaries, New Zealand

Spawning runs of brown trout Salmo trutta L., and rainbow trout, Oncorhynchus mykiss Walbaum, were compared in three tributaries of two lakes for timing, numbers and redd characteristics. Redd characteristics for the two species in four other streams were examined. No significant differences in redd characteristics between the two species were observed, but in all lake tributaries the possibilities of superimposition of spring spawning rainbow trout on redds of brown trout were examined. A common feature was the high ratio of numbers of spawners to the area of preferred spawning gravel, and it was concluded that in the lake tributaries, superimposition resulted in increased mortality of brown trout eggs, and in one case elimination of the brown trout population. Management possibilities for control of the species proportions in comparable situations are discussed.     

Spawning dynamics of American shad (Alosa sapidissima) in the St. Lawrence River,Canada–USA

Maltais E, Daigle G, Colbeck G, Dodson JJ. Spawning dynamics of American shad (Alosa sapidissima) in the St. Lawrence River, Canada–USA.
Ecology of Freshwater Fish 2010: 19: 586–594. © 2010 John Wiley & Sons A/S Abstract – The most northerly population of American shad (Alosa sapidissima), located in the St. Lawrence River, is considered vulnerable because of low population abundance and limited spawning habitat located at the upstream extent of the population’s anadromous migration. Here, we aimed to establish the temporal and spatial extent of spawning based on a novel hatch‐date analysis of juveniles. Spawning activity lasted from early May to early July. We found that juveniles captured downstream during the summer hatched later in the year than those captured further upstream. As a result, younger juveniles were distributed somewhat further downstream. In addition, we found significant multimodality in hatch‐date distributions at midstream and downstream sampling stations. Together, these results provide evidence that the 2‐month spawning period involved numerous spawning events that progressed in a downstream direction as the season advanced, rather than being restricted to upstream sites over the spawning season.     

Ichthyoplankton-based spawning dynamics of blue mackerel (Scomber australasicus) in south-eastern Australia: links to the East Australian Current

We describe findings of three ichthyoplankton surveys undertaken along south‐eastern Australia during spring (October 2002, 2003) and winter (July 2004) to examine spawning habitat and dynamics of blue mackerel (Scomber australasicus). Surveys covered ～860 nautical miles between southern Queensland (Qld; 24.6°S) and southern New South Wales (NSW; 41.7°S), and were mainly centred on the outer shelf including the shelf break. Egg identifications were verified applying mtDNA barcoding techniques. Eggs (n = 2971) and larvae (n = 727; 94% preflexion) occurred both in spring and winter, and were confined to 25.0–34.6°S. Greatest abundances (numbers per 10 m²) of eggs (1214–7390) and larvae (437–1172) occurred within 10 nm shoreward from the break in northern NSW. Quotient analyses on egg abundances revealed that spawning is closely linked to a combination of bathymetric and hydrographic factors, with the outer shelf as preferred spawning area, in waters 100–125 m deep with mean temperatures of 19–20°C. Eggs and larvae in spring occurred in waters of the East Australian Current (EAC; 20.6–22.3°C) and mixed (MIX; 18.5–19.8°C) waters, with none occurring further south in the Tasman Sea (TAS; 16.0–17.0°C). Results indicate that at least some of the south‐eastern Australian blue mackerel stock spawns during winter‐spring between southern Qld and northern NSW, and that no spawning takes place south of 34.6°S due to low temperatures (<17°C). Spawning is linked to the EAC intrusion, which also facilitates the southward transport of eggs and larvae. Since spring peak egg abundances came from where the EAC deflects offshore, eggs and larvae are possibly being advected eastwards along this deflection front. This proposition is discussed based on recent data on blue mackerel larvae found apparently entrained along the Tasman Front.     

Using underwater cameras to describe the reproductive behaviour of the endangered eastern freshwater cod Maccullochella ikei

Abstract –  Underwater cameras were used to observe the breeding behaviour of the endangered eastern freshwater cod, Maccullochella ikei , over 3 years and across three areas in the Mann and Nymboida rivers, Australia. The annual breeding season for M. ikei was short and succinct, lasting only 8–10 weeks. Spawning commenced each year in the lowest altitude area during the first week of spring, and approximately 1 week later in the closest upstream area. Day-length is the primary spawning cue for M. ikei , but increasing water temperature may also be of importance. Nesting sites were located in slow-flowing pools, under cover such as large boulders and bedrock shelves, at depths of 0.9–4.0 m, and with one or two entrances only. The nesting site was vigorously cleaned by the male up to 1 week prior to spawning and was only entered by the female for spawning. Paternal care of eggs and larvae was undertaken for up to 24 days, after which larvae dispersed. Greater protection of breeding M. ikei must be a management priority to ensure long-term conservation of the species.     

Timing of brown trout spawning in Alpine rivers with special consideration of egg burial depth

Timing of spawning, habitat use and egg burial depths of brown trout were studied in seven Swiss (alpine and prealpine) rivers, which differed in size, altitude and flow regime. In general, we observed brown trout spawning activity between the end of October and the beginning of January. The spawning duration differed significantly, however, between rivers, ranging from 28 to 72 days. Analysis of environmental parameters for their influence on spawning activity revealed mean water temperature and altitude as the most explanatory variables. Detailed investigation of redd characteristics based on water velocity, water depth and substrate size clearly showed differences between positions on the redd. Brown trout in Alpine rivers preferred to use velocities of 30–40 cm·s^?1, water depths of 10–20 cm and substrates of 16–32 mm for spawning. It has to be noted, however, that recorded values cover almost the whole range of data on spawning habitats that has been reported in literature so far. A special focus of this study was on egg burial depths, which were surprisingly not found to differ significantly between the rivers despite their different flow regimes. Recorded egg burial depths were, however, found to be distinctly lower (mean burial depth: 3.8 cm) than reported by almost any study so far. We see this observation of low burial depths in Alpine rivers as useful in the context of scouring effects, especially when evaluating the influence of scouring on fish populations.     

Dual effects of reversed winter–spring temperatures on year‐to‐year variation in the recruitment of chub mackerel (Scomber japonicus)

To clarify the effects of temperature on the recruitment of chub mackerel (Scomber japonicus) in the North Pacific, we investigated the influence of winter surface temperature (WST) on spawners at the time of maturity around the spawning grounds and the influence of ambient spring temperature on larvae using estimated temperature (ET) obtained from particle tracking experiments. We found a significant positive correlation between ET approximately 10 days following hatching and the recruitment per spawning stock biomass (RPS) after 2000. The closer (more meandering) the Kuroshio Current (KC) axis was in relation to the spawning ground, the higher (lower) the spring surface temperature and the higher (lower) RPS was in the spawning ground. In contrast, WST inside KC near the maturity/spawning ground was significantly negatively correlated with RPS. A significant negative correlation between the temperatures in winter and spring was detected in the area after 2000, when the conditions of the Pacific decadal oscillation index and the stability of the Kuroshio Extension were synchronous, indicating that KC shifted northward during this time. The reversed temperature pattern was consistent with the winter–spring movement of KC axis in the offshore direction and was correlated with the winter–spring difference in the intensity of the Aleutian low. These results suggest that the annual variation in chub mackerel recruitment after 2000 was strongly affected by the combined effects of ambient temperature because of the reversal of conditions that occurred between winter and spring around the maturity/spawning ground, which was related to the KC path.     

Improving spawning synchrony in the Nile tilapia, Oreochromis niloticus (L.)

Abstract. The development stage and number of seed (eggs and fry) removed from individual incubating Oreochromis niloticus (L.) broodfish at 10-day interharvest intervals (IHI) was compared with the harvest of free-swimming fry from undisturbed fish held in a tank within the same system. Spawning synchrony and productivity were compared for (1) total exchange of females for ‘conditioned’ females after each harvest, (2) exchange of only spawned females, and (3) return of females back into the spawning tank. Improved synchrony and seed production were positively related to removal of eggs and fry from incubating females and could have important implications for broodstock management in commercial hatcheries. Allowing fish to remain undisturbed in clear water tanks may lead to inhibition of spawning activity related to the formation of hierarchies. Individual weight loss by females during incubation was correlated with stage of seed harvested and thus with timing of spawning during the interharvest interval.     

Early out-of-season induced spawning of channel catfish Ictalurus punctatus (Rafinesque) conditioned in heated earthen ponds

This study documents early out-of-season induced spawning of channel catfish Ictalurus punctatus. During the early spring (February–April) of 1999, 2000 and 2001, ponds containing (1) male and female channel catfish (mixed-sex ponds) or (2) male channel and blue catfish I. furcatus only, or female channel catfish only (single-sex ponds) were heated to 24–30°C to encourage gonadal maturation and spawning. Unheated ponds were stocked with males and females and were monitored during the duration of heating. When natural spawning occurred in the heated ponds, the fish were captured by seining and unspawned females were injected with 100 μg kg⁻¹ of synthetic leutenizing hormone-releasing hormone. Injected females were either paired with males or held in communal all-female groups, and monitored for ovulation. Eggs were collected and fertilized with sperm of channel catfish or blue catfish. Females paired with males were induced to spawn 44 days (mixed-sex ponds) and 50 days (single-sex ponds) before natural spawning occurred in unheated ponds. Spawning latency (the time between injection and ovulation) and the percentage of neurulated embryos from eggs fertilized using channel catfish sperm was not different between spawning before the natural season (P=0.68) and during the natural season in fish from mixed-sex ponds (P=0.57). Females held in all-female groups produced eggs 34 days before the onset of spawning in unheated ponds. Spawning latency was not different between spawns before and during the natural season (P=0.16), and the percentages of neurulated embryos from eggs fertilized with channel catfish sperm (P=0.76) or blue catfish sperm (P=0.77) before or during the natural season were not different. This study demonstrates the feasibility of conditioning of channel catfish females for early out-of-season induced spawning in the laboratory.     

Effects of photoperiod and the duration of conditioning on gametogenesis and spawning of the mussel Mytilus galloprovincialis (Lamarck)

The mussel industry in Europe is facing two major problems: scarcity in wild seed supply in some regions and loss of commercial quality during the spawning season in other areas. The solution to these problems could be seed production in hatcheries. In addition, seed production in hatcheries would also allow the production of triploid individuals, with consequent benefits for the producer. This requires broodstock conditioning, to obtain mussel seed outside the natural season. In this study, the effects of photoperiod and the duration of the conditioning period were examined on the gametogenesis and spawning of the mussel Mytilus galloprovincialis (Lamarck) in a hatchery. The results obtained indicate that the conditioning of this mussel is possible in a hatchery by supplying a sufficient amount of food and increasing the water temperature. It was shown that gametogenesis of conditioned mussels was dependent on the duration of the conditioning period. Spawning, besides being dependent on the duration of the conditioning period, also appears to be dependent on photoperiod, although these results are not clear enough. This supports the hypothesis that gametogenesis and spawning are regulated by different physiological mechanisms.     

Spawning patterns provide further evidence for multiple stocks of sardine (Sardinops sagax) off eastern Australia

Previous studies have suggested that sardine, Sardinops sagax, off eastern Australia spawns across its entire range when habitat conditions are suitable. However, recent studies have suggested that separate sub‐populations and spawning groups may occur in the region. Spawning patterns off eastern Australia were investigated using data collected during nine ichthyoplankton surveys conducted between 1997–2015, and adult reproductive data obtained from ad hoc commercial sampling off New South Wales (NSW). The egg surveys covered the known distribution of sardine off eastern Australia and included year‐round sampling in the northern and southern parts of this range. Egg distributions and analysis of gonadosomatic indices identified two spatio‐temporally separate spawning groups; one occurring off southern Queensland to northern NSW during late winter and early spring, and a smaller group off eastern Tasmania to southern NSW during summer. Most eggs were collected from waters 50–90 m deep, with sea surface temperatures of 18–23°C. Additive modelling indicated depth was the most significant factor driving selection of spawning habitat, followed by the interaction of month and latitude. Low egg densities were recorded in waters between 34–37°S, despite conditions within the ranges suitable for spawning. The presence of two spawning groups of sardine off eastern Australia supports recent findings that two sub‐populations occur in the region. Findings of this study will help to optimise the spatio‐temporal extent of future egg surveys and further confirm the need to coordinate future management of each sub‐population among relevant jurisdictions.     

The ascent migration of Calanus finmarchicus from overwintering depths in the Faroe–Shetland Channel

The Faroe–Shetland Channel is an important overwintering area for the copepod Calanus finmarchicus in the north-east Atlantic. Stage V copepodites descend to depths of greater than 600 m, where they remain in an arrested development state (diapause) until the following spring. The ascent of the copepods back to the surface waters in the spring was investigated by six surveys in the Faroe–Shetland Channel between October 1993 and May 1995. In February, the first animals emerged from diapause and moulted to become stage VI males; these ascended to depths of 300–500 m. Females constituted a higher proportion of animals that emerged later in the season and these ascended through the layer of males, presumably mating on the way, and proceeded to the upper 100 m to begin spawning in March. Emergence was not synchronous in the population, and was estimated to occur over a 60–70 day period beginning around 10 February. The mean ascent rate of individuals was estimated to be 15–20 m day^–1, so that for any individual the ascent from a mean overwintering depth of 800 m to a depth of 100 m took between 35 and 47 days. The ascent migration resulted in the mass transfer of the C. finmarchicus stock from its overwintering habitat in the cold southerly flowing Norwegian Sea Deep Water of Arctic origin to warm northerly flowing Atlantic water masses. The extent of asynchrony suggests that emergence is probably not triggered by environmental cues. An alternative hypothesis for regulation of the life cycle is proposed that involves reduced development rates during the winter.     

Stability in reproductive timing and habitat usage of Chinook salmon across six years of varying environmental conditions and abundance

The spatial and temporal distribution of spawning activity by autumn‐run Chinook salmon Oncorhynchus tshawytscha (Walbaum) was examined across multiple years. The study period included two years of extreme drought conditions when water temperatures in the spawning reach of the Stanislaus River were considered sub‐optimal for spawning and egg incubation. Despite varying levels of superimposition, redd counts and densities remained stable and positively associated with river location, indicating that superimposition may be driven by habitat preference rather than the absence of suitable spawning locations. Spawning occurred slightly later during drought years (6–10 days later compared with wetter years). This delay was attributable to deferred migration instead of deferred spawning, and the majority of redds were constructed at water temperatures exceeding the optimal temperature range. As a consequence, estimates of juvenile production during 2014 and 2015 were among the lowest on record. These findings may be related in part to the high hatchery contribution to the population, above‐average temperatures during spawning and incubation, and superimposition rates. Management recommendations include adequate cold‐water storage in the upstream reservoir, refined spawning habitat restoration techniques in the light of superimposition rates and, on a broader system scale, actions that reduce the amount of stray hatchery Chinook salmon.     

Littoral spawning habitats of three southern Arctic charr (Salvelinus alpinus L.) populations

Abstract Arctic charr populations in southern latitudes are nonmigratory, with all life‐stages limited to freshwater lakes and in‐ or out‐flowing tributaries. Although many of these populations are reported to also spawn in lake littorals, little is known about the physical characteristics of putative spawning grounds. A total of 23 discrete spawning sites within three Irish lakes were located by fyke netting of spawning adults and snorkelling in littoral habitats. Spawning sites were found to be long, narrow strips running parallel to the shore at a maximum depth of 124 cm. Spawning sites were limited to areas of coarse mineral substrate with an adequate (c. 8 cm) depth of clean interstitial spaces. In individual lakes, combined areas of spawning sites made up 0.4–0.7% of available littoral. Egg densities varied considerably between sites (33–900·eggs m^?2) and were significantly correlated with gradient and width of spawning sites. No evidence of redd digging was found. The shallow, localised and restricted nature of spawning grounds makes such populations vulnerable to anthropogenically induced postoviposition changes in surface water level, eutrophication processes such as increased lake sedimentation and elevated nutrient status.     

Effect of habitat conditions on reproduction of the European anchovy (Engraulis encrasicolus) in the Strait of Sicily

The aims of this study were to describe the reproductive cycle of the European anchovy (Engraulis encrasicolus) off the south coast of Sicily and determine whether intra‐ and inter‐annual reproductive trait variations, if any, are adaptive responses which maximize reproductive success under environmental fluctuations. Biological data were collected from purse seine and mid‐water pelagic trawl commercial catches landed in Sciacca (Sicily) over 6 yr (1997–2002) at fortnightly intervals, analysing a total of 84 581 individuals. No inter‐annual changes in length at first reproduction were observed, with a mean pooled value of 11.26 cm for both sexes being found. Spawning intensity, indicated by gonadosomatic index, condition factor and length–weight relationships, seem to be governed by food availability prior to spawning. Anchovy reproductive investment was limited by the area's low primary production. There was a synchrony between reproductive cycle and temperature. Water warming marks the onset of a period of high water stability in the area, and its later cooling marks the onset of a period with low water stability. The relationship between reproductive cycle and temperature is therefore probably a reproductive strategy having evolved to ensure that spawning takes place during the period of the year when water column stability is higher, favouring food concentration and egg and larval retention in the spawning areas.     

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.