Tidal stream mediation of metapopulation structure in North Sea plaice

Migration is widespread among marine fishes, yet little is known about variation in the migration of individuals within localities. We tested the hypothesis that variation in the migratory behaviour among plaice (Pleuronectes platessa) in the North Sea could be explained by large‐scale differences in the speed and directions of the tidal streams, which the fish use as a transport mechanism. Towards this end, 752 mature female plaice tagged with electronic data storage tags were released at eight locations with contrasting tidal flow properties, between December 1993 and September 1999. The experiment yielded 20 403 days of data from 145 plaice. The position of each fish was determined at intervals throughout the liberty period using the tidal location method. The results show 3 geographically discrete feeding aggregations during the summer, which dispersed over the southern North Sea and Eastern English Channel to spawn during winter. Our results re‐affirmed the major role of the tidal streams in the southern North Sea in structuring plaice dispersion, both by providing transport and guidance, and by delimiting the extent of distribution due to thermal stratification during the summer. These results confirm the prediction that large‐scale variation in migration behaviour can be explained in part by the tidal guidance and transport mechanisms available. They have revealed features of spatial dynamics not previously observed from a century of conventional tagging experiments and illustrate how the study of individual fish can successfully define the migratory characteristics of populations.     

Being there and getting there: balancing the costs of migration against the benefits of changing habitat for North Sea plaice

To improve survival and reproductive success, many fish species have evolved migratory life‐histories, showing ontogenetic and/or seasonal changes in habitat use. Individuals move between different areas, each of which is 'best' for a particular activity, such as feeding, growing or spawning. The benefits of moving to a different habitat, however, have to be balanced against the costs of migration, so any behaviour that reduces the cost of movement would be expected to expand the migratory range and thereby increase the range of habitats that can be exploited.
Previous tracking experiments in the southern North Sea have shown that plaice selectively exploit the tidal streams to aid their spawning migration. Here we examine whether this behaviour is primarily (1) an energy‐saving strategy, or (2) a transport mechanism by which fish that are unable to navigate over long distances can be carried reliably between feeding and spawning grounds. Because selective tidal stream transport requires that fish remain stationary on the sea‐bed during the 'non‐transporting' tide, energetic calculations predict that this behaviour is beneficial only when the current speed exceeds a critical, size‐dependent, value. We have used detailed information about migratory behaviour from individual fish to calculate the metabolic costs and the likely reproductive benefits of different migratory strategies. Our results show that plaice use selective tidal stream transport only in areas where the tidal streams are suitably fast, indicating that this behaviour is primarily an energy saving strategy.     

The migratory behaviour of North Sea plaice: Currents, clocks and clues

Water currents are a dominant feature of the open sea and often play an important part in the life cycle of the fish that live there. For over three decades, a focus of research at Lowestoft has been the role of tidal water currents as a part of wider understanding of the population dynamics of plaice (Pleuronectes platessa) in the North Sea. Increasingly sophisticated telemetry methods have been developed and used, initially to describe the migratory behaviour of individual fish, and latterly to describe the spatial dynamics of whole populations. Here, the authors review cues and clues for environmental transport at different life stages, with a critical evaluation of the facultative use of selective tidal stream transport in relation to water current speed. The clues that trigger movement into mid-water by adult fish are reconsidered in light of new findings. Finally, the orientation capabilities of plaice, together with evidence of repeat migrations and spawning site fidelity were examined.     

The dispersal patterns and behaviour of North Sea cod (Gadus morhua) studied using electronic data storage tags*

The migratory behaviour of adult cod was studied using electronic data storage tags (DSTs). Fifty-eight cod were externally tagged with LOTEK LTD-100 DSTs and released off Lowestoft in March and April 1999. Twenty-two tags were returned by March 2000. Individuals demonstrated significant shifts in behaviour patterns that were associated with migration and residence on feeding grounds. During migration, individuals were very active and utilised a large vertical range. In contrast, individuals demonstrated low activity and a much reduced vertical range whilst on summer feeding grounds. Use of the Tidal Location Method showed that individuals dispersed to feeding grounds in the central and southern North Sea. Our results demonstrate that individual-based studies of fish behaviour can provide information relevant to fisheries management.     

Migration route and spawning area fidelity by North Sea plaice

Data from plaice, Pleuronectes platessa L., tagged with electronic data storage tags, were used to test whether these fishes exhibited migration route and spawning area fidelity in successive spawning seasons. Depth and temperature data were recorded for each fish over 365-512 days in the central North Sea and this information was used to reconstruct movements based on tidal locations. We discovered highly directed seasonal migrations from the winter spawning area south of a major topographical feature, Dogger Bank Tail End, to summer feeding grounds 250 km to the north in deep, cold, thermally stratified water. Our results show synchronous timing of migration, repeated pre- and post-spawning migration routes and 100% spawning area fidelity, including two individuals that returned to within 20 km of their previous season's spawning location. This is the first study to provide a complete reconstruction of annual migrations by individual fishes, showing strong homing behaviour along consistent migration routes.     

Seasonal Ichthyodiversity and Growth Patterns of Juvenile Flatfish on a Nursery Ground in the Southern Bight of the North Sea (France)

The demersal fish community on a flatfish nursery ground, of the Southern Bight of the North Sea, was sampled monthly between May 1998 and 1999. The studied coastal area is a multispecific nursery area. Although 32 fish species were caught, only nine species had a major influence on the variation in total densities through the year and can be considered as key species. Juvenile sole and plaice exhibited similar seasonal growth patterns with rapid growth during late spring and summer, a growth arrest during autumn and winter and a restart of growth in March. Comparisons of observed growth in length with predicted maximal growth under optimal food conditions suggested that during the summer, growth of 0-group sole and plaice was only determined by prevailing mean water temperature. During autumn and winter, while growth in length of sole followed model predictions, observed length of plaice was less than model predictions, suggesting growth limitation. Analyses of the factors that may be responsible for differences between observed and expected length growth indicated that autumn and winter growth arrest of plaice was not only related to low winter water temperature. It is suggested that for visual feeders such as plaice, the interaction of decreasing food availability and day length during autumn and winter can reduce the access to food resource and therefore feeding success and growth.     

Postglacial northward expansion and genetic differentiation between migratory and sedentary populations of the broad‐tailed hummingbird (Selasphorus platycercus)

Unlike other migratory hummingbirds in North America, the broad‐tailed hummingbird (Selasphorus platycercus) exhibits both long‐distance migratory behaviour in the USA and sedentary behaviour in Mexico and Guatemala. We examined the evolution of migration linked to its northward expansion using a multiperspective approach. We analysed variation in morphology, mitochondrial and nuclear DNA, estimated migration rates between migratory and sedentary populations, compared divergence times with the occurrence of Quaternary climate events and constructed species distribution models to predict where migratory and sedentary populations resided during the Last Glacial Maximum (LGM) and Last Interglacial (LIG) events. Our results are consistent with a recent northward population expansion driven by migration from southern sedentary populations. Phylogeographical analyses and population genetics methods revealed that migratory populations in the USA and sedentary populations in Mexico of the platycercus subspecies form one admixed population, and that sedentary populations from southern Mexico and Guatemala (guatemalae) undertook independent evolutionary trajectories. Species distribution modelling revealed that the species is a niche tracker and that the climate conditions associated with modern obligate migrants in the USA were not present during the LIG, which provides indirect evidence for recent migratory behaviour in broad‐tailed hummingbirds on the temporal scale of glacial cycles. The finding that platycercus hummingbirds form one genetic population and that suitable habitat for migratory populations was observed in eastern Mexico during the LIG also suggests that the conservation of overwintering sites is crucial for obligate migratory populations currently facing climate change effects.     

A quantitative description of the behaviour of wild juvenile plaice (Pleuronectes platessa L.)

The behaviour of young plaice (Pleuronectes platessa L.) as they migrate up and down sandy beaches with the tide is described. Their behaviour during this migration consists mainly of swimming and feeding interspersed with rarer behavioural acts. Over short periods swimming behaviour can be described by a random model in which the probability of a swim occurring remains constant. This probability varies markedly, however, from hour to hour. Two types of swimming movement are recognized: one of very short duration represents searching for food and the other, longer, category serves to transport the fish up and down the shore. Variations in the feeding rate and in the frequency, duration, and direction of swimming movements over the tidal cycle are described and related to the changing physical and biological conditions that the fish experience during their intertidal movements.     

Recent trends in the abundance of plaice Pleuronectes platessa and cod Gadus morhua in shallow coastal waters of the Northeastern Atlantic continental shelf – a review

Shallow, near-shore water habitats on the continental shelf of the Northeast Atlantic have been productive fishing areas in the past. Here, we review the present knowledge about (i) recent trends in the abundance of plaice and cod in these habitats and (ii) hypotheses regarding the factors responsible for any trends. At present, only a few studies exist on the trends of abundance of plaice or cod, namely from the Bay of Biscay, the North Sea and the Skagerrak/Kattegat. They suggest a declining abundance in coastal, shallow areas and – at least for plaice – a latitudinal gradient with an erosion of the southern distribution boundary in the Bay of Biscay and deepening of stocks in the North Sea. In contrast, no trend in shallow water abundance of plaice similar to a decline in deep-water stocks during the 1970s and their slow recovery during the 2000s is apparent in the Skagerrak/Kattegat. Although shallow habitats fundamentally differ from deeper areas by the prevalence of juvenile stages, the declining trends coincide with decreasing abundance/landings and spatial stock relocations in the deeper areas. Whether this indicates a common trend pointing at connectivity between shallow and deep water remains open. Fundamental differences exist in the suggested causes of the trends in different geographical areas. High fishing pressure together with low local recruitment apparently prevents the recovery of overexploited plaice and cod stocks in the Skagerrak/Kattegat. In contrast, the responses of juveniles and adult fish to increasing seawater temperature are the main hypotheses for changes in distribution and abundance of both fish species in the North Sea/Bay of Biscay. However, temperature alone cannot explain the observed decline of fish in coastal areas, and the causes may be more complex, involving nutrient loading, primary productivity or food availability, although at present, knowledge of these factors is insufficient.     

An optimal foraging and migration model for juvenile plaice

Summary A state-dependent model has been used to predict daily and tidal patterns of migration, feeding and inactivity in juvenile plaice (Pleuronectes platessa L.) in their intertidal and shallow subtidal nursery areas. Vertical position, quantity of energy reserves and fullness of the gut characterized the state of individual fishes. If feeding is visually mediated, the model predicts that, by night, plaice should move to areas of low predation risk and become inactive, whereas by day, plaice should migrate to feed in areas of high prey encounter rate. Vertical zones adopted by day and night and, hence, patterns of migration, should depend on the distributions of predators and prey. When prey are more abundant in the intertidal zone, plaice should move onshore to feed as the tide rises and when prey are more abundant offshore, plaice should move offshore to feed by day. If predators are equally abundant in all zones, the fish should behave as if no predators were present, having no effective refuge. An increase in the abundance of predators with depth results in the restriction of plaice activities to shallower vertical zones, depending on the magnitude of the predation threat. Zones adopted thus depend on the trade-off between energy intake and predation risk. Concordance between predicted behaviour and observed patterns is evident in contrasting habitats. Migration and feeding in the Wadden Sea, where prey are more abundant on intertidal flats, is dominated by the tidal component, whereas on impoverished exposed beaches of the west coast of Scotland, the diurnal component is dominant. Tidally related behaviour persists in the latter environment, not predicted by the model and may be a consequence of using endogenous rhythms to approach optimal behavioural patterns.     

Escaping peril: perceived predation risk affects migratory propensity

Although migratory plasticity is increasingly documented, the ecological drivers of plasticity are not well understood. Predation risk can influence migratory dynamics, but whether seasonal migrants can adjust their migratory behaviour according to perceived risk is unknown. We used electronic tags to record the migration of individual roach (Rutilus rutilus), a partially migratory fish, in the wild following exposure to manipulation of direct (predator presence/absence) and indirect (high/low roach density) perceived predation risk in experimental mesocosms. Following exposure, we released fish in their lake summer habitat and monitored individual migration to connected streams over an entire season. Individuals exposed to increased perceived direct predation risk (i.e. a live predator) showed a higher migratory propensity but no change in migratory timing, while indirect risk (i.e. roach density) affected timing but not propensity showing that elevated risk carried over to alter migratory behaviour in the wild. Our key finding demonstrates predator-driven migratory plasticity, highlighting the powerful role of predation risk for migratory decision-making and dynamics.     

Multi-torsking: simultaneous measurements of cod behaviour show differences between North Sea and Irish Sea stocks*

Fish behaviour can vary considerably in response to environmental and biological conditions. We used electronic Data Storage Tags (DSTs) to investigate the behaviour of cod in different environments; 74 North Sea cod and 20 Irish Sea cod were tagged and released in late-March 1999. So far, 32 tags (>2000 days of data) have been returned from the North Sea and four (>700 days) from the Irish Sea. The maximum recorded depth of a North Sea cod was 84 m, while Irish Sea cod utilised a depth range of 154 m. Active depth changes were of greater magnitude and rate in Irish Sea cod than North Sea cod. In addition, Irish Sea cod were continually active and made proportionally more active depth changes than North Sea cod, which were very active during migration (March–June) but exhibited little vertical movement during the summer months. To our knowledge, this is the first time that the behaviour patterns of cod from different stocks have been measured simultaneously over extended periods at such a fine temporal scale. In addition to describing considerable inter-stock differences in behaviour, our results suggest that while cod in the Irish Sea remain active and highly mobile throughout the year, North Sea cod reduce their movements during the summer feeding period. Our findings will contribute to the development of more biologically realistic management models used for evaluating the efficacy of fisheries technical measures, such as closed areas.     

Analysis of the hatching site and migratory behaviour of the swordtip squid (Uroteuthis edulis) caught in the Japan Sea and Tsushima Strait in autumn estimated by statolith analysis

In contrast to the swordtip squid (Uroteuthis edulis) caught in the spring and summer, those caught in the autumn in the Japan Sea and the Tsushima Strait are characterized by thick bodies with large clubs, long tentacles and large suckers. In addition, most females and many males are immature. However, the reasons for these characteristics, as well as the hatching site and migratory behaviour, are not well known. The empirical water temperature of juveniles was estimated at ～17°C through strontium:calcium (Sr:Ca) ratios in statoliths and information on seawater temperature distribution. The ontogenetic variation in Sr:Ca ratios decreased from hatching to 60 days after hatching and then remained stable. The estimated water temperature was ～21°C after 60 days. Thus, individuals of the autumn-migrating group most likely hatch in the southern East China Sea in the winter and early spring and then move north-eastwards with currents and pass through the Tsushima Strait into the southern Japan Sea during or after July. However, the squid could be present in these waters without strong currents northeast of Tsushima Island, where thermoclines occur during summer, and become confined in limited layers until vertical mixing occurs in the autumn. The distribution of the squid would thereafter extend to fishing grounds in the southern and south-eastern Japan Sea and then to the Tsushima Strait. This migratory behaviour could be the cause of the observed characteristics because bottom seawater temperatures are much lower in the Japan Sea than in the East China Sea.     

The intertidal movements and distribution of young fish on a sandy beach with special reference to the plaice (Pleuronectes platessa L.)

The intertidal movements of young fish were studied on a sandy beach over several tidal cycles during the day and the night. Young plaice (Pleuronectes platessa L.), by virtue of their abundance, received the most attention, and it was found that they migrate up and down the beach with each tide in company with several other less abundant species. The dab (Limanda limanda (L.)), does not make this migration.The vertical distribution of the commoner fish species was charted and each was found to occupy a specific zone in relation to the water, but with large overlaps in many cases. A close relationship between the length of plaice and dabs and the depth in which they were caught was evident during the summer months, but this relationship broke down in the winter due mainly to an offshore movement of larger fish.The food intake of plaice was studied throughout the day and night; feeding occurred mostly during the day. No effect of the tide on feeding could be detected.The possible mechanisms controlling the tide-related movements and the size-depth relationship are discussed and a general hypothesis is put forward in which it is proposed that the fish are able to select and maintain their position at a particular depth throughout a tidal cycle thereby being guided up and down the beach. Attributes of depth which the fish might use as a reference point in their movements are discussed.     

Listening In on the Past: What Can Otolith δ18O Values Really Tell Us about the Environmental History of Fishes?

Oxygen isotope ratios from fish otoliths are used to discriminate marine stocks and reconstruct past climate, assuming that variations in otolith δ¹⁸O values closely reflect differences in temperature history of fish when accounting for salinity induced variability in water δ¹⁸O. To investigate this, we exploited the environmental and migratory data gathered from a decade using archival tags to study the behaviour of adult plaice (Pleuronectes platessa L.) in the North Sea. Based on the tag-derived monthly distributions of the fish and corresponding temperature and salinity estimates modelled across three consecutive years, we first predicted annual otolith δ¹⁸O values for three geographically discrete offshore sub-stocks, using three alternative plausible scenarios for otolith growth. Comparison of predicted vs. measured annual δ¹⁸O values demonstrated >96% correct prediction of sub-stock membership, irrespective of the otolith growth scenario. Pronounced inter-stock differences in δ¹⁸O values, notably in summer, provide a robust marker for reconstructing broad-scale plaice distribution in the North Sea. However, although largely congruent, measured and predicted annual δ¹⁸O values of did not fully match. Small, but consistent, offsets were also observed between individual high-resolution otolith δ¹⁸O values measured during tag recording time and corresponding δ¹⁸O predictions using concomitant tag-recorded temperatures and location-specific salinity estimates. The nature of the shifts differed among sub-stocks, suggesting specific vital effects linked to variation in physiological response to temperature. Therefore, although otolith δ¹⁸O in free-ranging fish largely reflects environmental temperature and salinity, we counsel prudence when interpreting otolith δ¹⁸O data for stock discrimination or temperature reconstruction until the mechanisms underpinning otolith δ¹⁸O signature acquisition, and associated variation, are clarified.     

长山列岛邻近海域鱼类群落种类组成和多样性时空变化

为了解黄、渤海生态系统交错带长山列岛邻近水域鱼类群落种类组成和多样性,根据2016年10月,2017年1月、5月及8月进行的鱼类资源底拖网调查数据,应用相对重要性指数、物种多样性指数、k优势度曲线等方法,研究了长山列岛邻近海域鱼类群落种类组成、物种多样性时空变化及其与环境因子的关系。结果表明: 该海域共计捕获鱼类77种,以温水性、底层、洄游性鱼类为主,优势种组成季节变化明显,春季、冬季主要优势种为黄鮟鱇,夏季以日本鲭、鳀等中上层鱼类为主。全年共计出现洄游性鱼类46种,季节间物种迁移指数均在100以上,其中秋季物种迁移指数最大。鱼类群落物种丰富度指数在春季最高、Shannon多样性指数和均匀度指数在秋季最高。夏季物种丰富度指数与表层水温呈显著负相关;冬季物种丰富度指数与水深、底层水温呈极显著正相关,Shannon多样性指数与底层水温呈极显著正相关。长山列岛邻近海域作为黄、渤海两大生态系统的交错带,鱼类群落表现出高物种多样性、洄游种多以及明显的时空异质性。     

Associations between host migration and the prevalence of a protozoan parasite in natural populations of adult monarch butterflies

1. Monarch butterflies Danaus plexippus (L.) (Lepidoptera: Nymphalidae) are susceptible to infection by the obligate protozoan parasite Ophryocystis elektroscirrha (McLaughlin and Myers) (Apicomplexa: Neogregarinida). Because monarchs form resident and migratory populations in different parts of the world, this host–parasite system provides the opportunity to examine how variation in parasite prevalence relates to host movement patterns. 2. Parasite prevalence was evaluated using 14 790 adult monarchs captured between 1968 and 1997. Comparison of three populations in North America indicated that parasite prevalence is associated negatively with host dispersal distances. A continuously breeding, nonmigratory population in southern Florida showed high prevalence (over 70% heavily infected). The western population migrates moderate distances to overwintering sites on the Pacific Coast and has intermediate prevalence (30% heavily infected). The eastern migratory population, which travels the longest distance to Mexican overwintering sites, has exhibited less than 8% infection throughout the past 30 years. 3. Variation in parasite loads within North American migratory populations was investigated to determine whether the prevalence of heavy infection and average parasite loads declined during migration or overwintering. Average parasite loads of summer‐breeding adults in western North America decreased with increasing distance from overwintering sites. This suggests that heavily infected monarchs are less likely to remigrate long distances in spring. No differences in the frequency of heavily infected adults were found among eastern or western North American monarchs throughout the overwintering period, however, suggesting that this parasite does not affect overwintering mortality. 4. Changes in the prevalence of monarchs with low parasite loads demonstrate that spore transfer occurs during migration and overwintering, possibly when adult butterflies contact each other as a result of their clustering behaviour. 5. This study of geographical and temporal variation in O. elektroscirrha among populations of D. plexippus demonstrates the potential role of seasonal migration in mediating interactions between hosts and parasites, and suggests several mechanisms through which migratory behaviour may influence parasite prevalence.     

Evidence of multiple migrations between freshwater and marine habitats of Salvelinus leucomaenis

The migratory history of the white‐spotted charr Salvelinus leucomaenis was examined using otolith microchemical analysis. The fish migrated between freshwater and marine environments multiple times during their life history. Some white‐spotted charr used an estuarine habitat prior to smolting and repeated seaward migration within a year.     

Polyculture production of juvenile fishes for survival in nature

Anecdotal and empirical evidence exists for substantial (up to 40%) declines in growth among Oreochromis populations domesticated in both large and small‐scale fish farms in Africa. These declines are at least partly attributable to poor genetic management, including inadvertent selection, inbreeding, bottle‐necks and founder effects. Due to restricted cash flow and investment capital, genetic management and selective breeding for the improvement of domesticate populations are difficult for small‐scale farmers, but feasible on larger‐scale farms. In managing domesticated gene pools, feral populations can serve as a broodstock reservoir, making the use of indigenous species advantageous. A development model of large‐scale hatcheries producing selected lines of sex‐reversed, indigenous tilapia for sale to smaller‐scale farmers is proposed as a solution to the problems of poor genetic management in African aquaculture.     

Effects of discharge and local density on the growth of juvenile Atlantic salmon Salmo salar

The study explored the combined effects of density, physical habitat and different discharge levels on the growth of juvenile Atlantic salmon Salmo salar in artificial streams, by manipulating flow during both summer and winter conditions. Growth was high during all four summer trials and increased linearly with discharge and mean velocity. Differences in fish densities (fish m^?3) due to differences in stream volume explained a similar proportion of the variation in mean growth among discharge treatments. Within streams, the fish aggregated in areas of larger sediment size, where shelters were probably abundant, while growth decreased with increasing densities. Fish appeared to favour the availability of shelter over maximization of growth. Mean growth was negative during all winter trials and did not vary among discharge treatments. These results suggest that increased fish densities are a major cause of reduced summer growth at low discharge, and that habitat‐mediated density differences explain the majority of the growth variation across habitat conditions both during summer and winter.