Inter‐annual variation in herring,Clupea harengus,and sprat,Sprattus sprattus,condition in the central Baltic Sea: what gives the tune?

The Baltic Sea ecosystem has undergone large changes during the last two decades, including a severe reduction in cod and herring biomass but, at the same time, a large increase in sprat abundance. The lower trophic levels of the Baltic Sea also changed due to environmental fluctuations, including variations in salinity and in volume of oxygenated water. In this apparently shifting environment, the conditions of herring and sprat have undergone large inter-annual variations during the past 15–20 years. In this study, we explore how abiotic factors (i.e. salinity and temperature) and biotic factors (biomass of the copepods Pseudocalanus elongatus , Temora longicornis , Acartia spp. and of cladocerans as well as clupeid abundance) in different seasons (May and August) affect clupeid body condition. Our analyses suggest that data of zooplankton biomass and abiotic factors in August have higher predictive power than May data. Although our analysis suggests that salinity (a bottom-up process) has an effect on sprat condition, total abundance of clupeids (a top-down process) is by far the most significant predictor of both herring and sprat condition. The strong correlation between clupeid abundance and total zooplankton biomass points to food competition and to top-down control by herring and sprat on common food resources. Furthermore, clupeid condition co-varied with the changes in the weight of zooplankton in the stomachs, which further suggest food competition being the main mechanism behind the changes in clupeid condition during the last two decades. Hence, our results are not in agreement with most of the current literature that has suggested that clupeid growth is regulated by environmentally mediated bottom-up processes acting on the abundance of copepods. This is, to our knowledge, the first evidence of food resources mediated density-dependent fish growth in a large marine ecosystem.     

Population dynamics of calanoid copepods and the implications of their predation by clupeid fish in the Central Baltic Sea

Population dynamics of major Baltic calanoid copepod speciesin the Gotland Basin during the last two decades were characterizedby a decline of Pseudocalanus elongatus associated with decliningsalinities, and an increase of Temora longicornis and Acartiaspp. potentially due to warmer conditions. Additionally thisstudy investigated the effect of predation by the major planktivorousfish species herring (Clupea harengus) and sprat (Sprattus sprattus)for the period 1977–1996 in the Gotland Basin (CentralBaltic Sea). Examination of consumption by these fish speciesin relation to copepod production estimates showed a switchby herring from consuming mainly CV/VI of P. elongatus and T.longicornis, to preying on CII of the latter copepod. This switchwas potentially due to increased competition with the drasticallyincreased sprat stock since the late 1980s. Further, an increasedpredation pressure by sprat on CV/CVI of both copepod speciesin spring resulted in higher copepod mortality rates. In consequence,based on these results we suggest that the increase in the spratstock since the late 1980s contributed to a decline of P. elongatus,and additionally prevented an even more pronounced temperature-drivenincrease in the T. longicornis stock, as was observed for Acartiaspp., which was not significantly consumed.     

Seasonal variation in egg morphology and hatching success in the calanoid copepods Temora longicornis, Acartia clausi and Centropages hamatus

We report here on the seasonal changes in ultramicroscopic morphologyof the eggs of the calanoid copepod Temora longicornis. Duringa field study between 1996 and 1998 in the Menai Strait, easternIrish Sea, T. longicornis produced two types of eggs; hatchingor subitaneous eggs were spawned during autumn–winter,whereas a high proportion of non-hatching eggs (up to     

On the morphology and life-history of Hemiurus luehei Odhner, 1905 (Digenea: Hemiuridae)

Previously undescribed cystophorous cercariae which develop in sporocyst germinal sacs in the tectibranch opisthobranch Philine denticulata (Adams) are shown to be cercariae of Hemiurus luehei Odhner, 1905 (Hemiuridae), a common stomach parasite of clupeid and salmonid fishes off the Atlantic coast of Europe, in the Baltic and the Mediterranean Sea. The free-swimming cercariae are seized by calanoid copepods. Temora longicornis (Müller) and Acartia tonsa Dana acted as suitable experimental intermediate hosts. Pressure by the copepod mouth appendages causes delivery tube eversion and the injection of the cercarcial body into the haemocoel. Sagitta sp. was found naturally infected with a metacercaria of H. luehei. The cystophorous cercaria and metacercaria are described using light and scanning electron microscopy. Adults from herring were examined under the stereoscan electron microscope.     

Long-term dynamics of main mesozooplankton species in the central Baltic Sea

Long-term dynamics (1959–1997) of the copepod speciesPseudocalanus elongatus, Temora longicornis, Acartia spp. andCentropages hamatus, as well as the taxonomic group of cladocerans,are described for the open sea areas of the central Baltic Sea.Differences between areas, i.e. Bornholm Basin, Gdansk Deepand Gotland Basin, as well as between 5 year periods, were investigatedby means of Analysis of Variance (ANOVA). No significant differencesin mesozooplankton biomass between areas were found. On theother hand, clear time-trends could be demonstrated and relatedto salinity and temperature, with P.elongatus biomass mainlydependent on salinity and T.longicornis, Acartia spp. and cladoceransbiomasses dependent, to a large extent, on thermal conditions.Decreasing salinities since the early 1980s due to a lack ofmajor inflows of highly saline water from the North Sea andincreased river run-off, both triggered by meteorological conditions,obviously caused a decrease in biomass of P.elongatus. Contrarily,the standing stocks of the other abundant copepod species andcladocerans followed, to a large degree, the temperature developmentand showed, in general, an increase. The shift in species compositionduring this period is considered to be a reason for decreasinggrowth rates of Baltic herring (Clupea harengus) since the early1980s, and for sprat (Sprattus sprattus) since the early 1990s.Generally, it is suggested that low mesozooplankton biomassesin the 1990s were caused, at least partially, by amplified predationby clupeid fish stocks.     

Long-term dynamics of zooplankton in the southeastern Baltic Sea

Long-term research in the Baltic Sea revealed the basic trends of zooplankton community variations depending on oceanographic processes. Alternation of the periods of increase and decrease in salinity of the Baltic Sea against the background of climate changes (temperature increase) and eutrophication affect the state of the entire Baltic ecosystem, including zooplankton. For these periods, the dynamics of zooplankton in the Baltic Sea were analyzed based on literature data and results of regular research in the southeastern Baltic Sea during 1998–2007. The changes in the hydrological situation were accompanied by significant changes in the zooplankton community. In the 1990s–2000s, the abundance and biomass of brackish-water and thermophilous species primarily of Cladocera and Copepoda increased markedly. The role of the previously dominant marine copepod Pseudocalanus elongatus decreased due to salinity reduction in the deep-water part of the Baltic Sea. Maximum development of zooplankton occurred in years of the greatest warming-up of the water (2001, 2005–2007) against the background of a general positive trend of zooplankton abundance in the last decade.     

A Less Saline Baltic Sea Promotes Cyanobacterial Growth,Hampers Intracellular Microcystin Production,and Leads to Strain-Specific Differences in Allelopathy

Salinity is one of the main factors that explain the distribution of species in the Baltic Sea. Increased precipitation and consequent increase in freshwater inflow is predicted to decrease salinity in some areas of the Baltic Sea. Clearly such changes may have profound effects on the organisms living there. Here we investigate the response of the commonly occurring cyanobacterium Dolichospermum spp. to three salinities, 0, 3 and 6. For the three strains tested we recorded growth, intracellular toxicity (microcystin) and allelopathic properties. We show that Dolichospermum can grow in all the three salinities tested with highest growth rates in the lowest salinity. All strains showed allelopathic potential and it differed significantly between strains and salinities, but was highest in the intermediate salinity and lowest in freshwater. Intracellular toxin concentration was highest in salinity 6. In addition, based on monitoring data from the northern Baltic Proper and the Gulf of Finland, we show that salinity has decreased, while Dolichospermum spp. biomass has increased between 1979 and 2013. Thus, based on our experimental findings it is evident that salinity plays a large role in Dolichospermum growth, allelopathic properties and toxicity. In combination with our long-term data analyses, we conclude that decreasing salinity is likely to result in a more favourable environment for Dolichospermum spp. in some areas of the Baltic Sea.     

Diversity and abundance of freshwater Actinobacteria along environmental gradients in the brackish northern Baltic Sea

Actinobacteria are highly abundant in pelagic freshwater habitats and also occur in estuarine environments such as the Baltic Sea. Because of gradients in salinity and other environmental variables estuaries offer natural systems for examining factors that determine Actinobacteria distribution. We studied abundance and community structure of Bacteria and Actinobacteria along two transects in the northern Baltic Sea. Quantitative (CARD-FISH) and qualitative (DGGE and clone libraries) analyses of community composition were compared with environmental parameters. Actinobacteria accounted for 22–27% of all bacteria and the abundance changed with temperature. Analysis of 549 actinobacterial 16S rRNA sequences from four clone libraries revealed a dominance of the freshwater clusters ac I and ac IV, and two new subclusters ( ac I-B scB-5 and ac IV-E) were assigned. Whereas ac I was present at all stations, occurrence of ac II and ac IV differed between stations and was related to dissolved organic carbon (DOC) and chlorophyll a (Chl a ) respectively. The prevalence of the ac I-A and ac I-B subclusters changed in relation to total phosphorus (Tot-P) and Chl a respectively. Community structure of Bacteria and Actinobacteria differed between the river station and all other stations, responding to differences in DOC, Chl a and bacterial production. In contrast, the composition of active Actinobacteria (analysis based on reversely transcribed RNA) changed in relation to salinity and Tot-P. Our study suggests an important ecological role of Actinobacteria in the brackish northern Baltic Sea. It highlights the need to address dynamics at the cluster or subcluster phylogenetic levels to gain insights into the factors regulating distribution and composition of Actinobacteria in aquatic environments.     

Salinity gradient of the Baltic Sea limits the reproduction and population expansion of the newly invaded comb jelly Mnemiopsis leidyi

The recent invasion of the comb jelly Mnemiopsis leidyi into northern European waters is of major public and scientific concern. One of the key features making M. leidyi a successful invader is its high fecundity combined with fast growth rates. However, little is known about physiological limitations to its reproduction and consequent possible abiotic restrictions to its dispersal. To evaluate the invasion potential of M. leidyi into the brackish Baltic Sea we studied in situ egg production rates in different regions and at different salinities in the laboratory, representing the salinity gradient of the Baltic Sea. During October 2009 M. leidyi actively reproduced over large areas of the Baltic Sea. Egg production rates scaled with animal size but decreased significantly with decreasing salinity, both in the field (7-29) and in laboratory experiments (6-33). Temperature and zooplankton, i.e. food abundance, could not explain the observed differences. Reproduction rates at conditions representing the Kattegat, south western and central Baltic Sea, respectively, were 2.8 fold higher at the highest salinities (33 and 25) than at intermediate salinities (10 and 15) and 21 times higher compared from intermediate to the lowest salinity tested (6). Higher salinity areas such as the Kattegat, and to a lower extent the south western Baltic, seem to act as source regions for the M. leidyi population in the central Baltic Sea where a self-sustaining population, due to the low salinity, cannot be maintained.     

Influence of oceanic factors on Anguilla anguilla (L.) over the twentieth century in coastal habitats of the Skagerrak,southern Norway

The European eel (Anguilla anguilla L.) is distributed in coastal and inland habitats all over Europe, but spawns in the Sargasso Sea and is thus affected by both continental and oceanic factors. Since the 1980s a steady decline has been observed in the recruitment of glass eels to freshwater and in total eel landings. The eel is considered as critically endangered on the International Union for the Conservation of Nature and Natural Resources Red List of species. The Skagerrak beach seine survey from Norway constitutes the longest fishery-independent dataset on yellow/silver eels (starting in 1904). The Skagerrak coastal region receives larvae born in the Sargasso Sea spawning areas that have followed the Gulf Stream/North Atlantic Drift before they penetrate far into the North Sea. The Skagerrak coastal time series is therefore particularly valuable for exploring the impacts of oceanic factors on fluctuations in eel recruitment abundance. Analyses showed that Sargasso Sea surface temperature was negatively correlated with eel abundance, with a lag of 12 years revealing a cyclic and detrimental effect of high temperatures on the newly hatched larvae. The North Atlantic Oscillation index and inflow of North Atlantic water into the North Sea were negatively correlated with eel abundance, with a lag of 11 years. Increased currents towards the North Atlantic during high North Atlantic Oscillation years may send larvae into the subpolar gyre before they are ready to metamorphose and settle, resulting in low recruitment in the northern part of the distribution area for these years. The Skagerrak time series was compared with glass eel recruitment to freshwater in the Netherlands (Den Oever glass eel time series), and similar patterns were found revealing a cycle linked to changes in oceanic factors affecting glass eel recruitment. The recent decline of eels in the Skagerrak also coincided with previously documented shifts in environmental conditions of the North Sea ecosystem.     

Scenario simulations of future salinity and ecological consequences in the Baltic Sea and adjacent North Sea areas–implications for environmental monitoring

Substantial ecological changes occurred in the 1970s in the Northern Baltic during a temporary period of low salinity (S). This period was preceded by an episodic increase in the rainfall over the Baltic Sea watershed area. Several climate models, both global and regional, project an increase in the runoff of the Northern latitudes due to proceeding climate change. The aim of this study is to model, firstly, the effects on Baltic Sea salinity of increased runoff due to projected global change and, secondly, the effects of salinity change on the distribution of marine species. The results suggest a critical shift in the S range 5–7, which is a threshold for both freshwater and marine species distributions and diversity. We discuss several topics emphasizing future monitoring, modelling, and fisheries research. Environmental monitoring and modelling are investigated because the developing alternative ecosystems do not necessarily show the same relations to environment quality factors as the retiring ones. An important corollary is that the observed and modelled S changes considered together with species’ ranges indicate what may appear under a future climate. Consequences could include a shift in distribution areas of marine benthic foundation species and some 40–50 other species, affiliated to these. This change would extend over hundreds of kilometres, in the Baltic Sea and the adjacent North Sea areas. Potential cascading effects, in coastal ecology, fish ecology and fisheries would be extensive, and point out the necessity to develop further the “ecosystem approach in the environmental monitoring”.     

Experimental insights into the importance of ecologically dissimilar bacteria to community assembly along a salinity gradient

The response of local communities to marine–freshwater transitions and the processes that underlie community assembly are unclear, particularly with respect to bacteria that differ in their life strategies. Here, we implemented a transplant experiment where bacterioplankton from three regions of the Baltic Sea with differing salinities (～3, 7 and 28 psu) were exposed to each other's environmental conditions. We found that habitat specialists were more abundant than generalists after exposure to salinity changes, irrespective of their origins. Most specialists that were selected following a salinity change were rare in the starting communities. Selection for generalists, however, was not specifically driven by the recruitment of either rare or abundant members, suggesting that taxon's initial abundance is minor relevant to the growth of generalists. Patterns in phylogenetic relatedness indicated that environmental filtering was the most influential assembly mechanism for specialists, whereas competitive interaction was more important for the assembly of generalists. Altogether, this study shows that large salinity changes promote the establishment of habitat specialists and that deterministic processes vary during community assembly for ecologically dissimilar taxa. We, therefore, propose that distinguishing assembly mechanisms of different community members helps understand and predict community dynamics in response to environmental change.     

Paralytic shellfish toxins or spirolides? The role of environmental and genetic factors in toxin production of the Alexandrium ostenfeldii complex

Dinoflagellates of the Alexandrium ostenfeldii complex (A. ostenfeldii, A. peruvianum) are capable of producing different types of neurotoxins: paralytic shellfish toxins (PSTs), spirolides and gymnodimines, depending on the strain and its geographic origin. While Atlantic and Mediterranean strains have been reported to produce spirolides, strains originating from the brackish Baltic Sea produce PSTs. Some North Sea, USA and New Zealand strains contain both toxins. Causes for such intraspecific variability in toxin production are unknown. We investigated whether salinity affects toxin production and growth rate of 5 A. ostenfeldii/peruvianum strains with brackish water (Baltic Sea) or oceanic (NE Atlantic) origin. The strains were grown until stationary phase at 7 salinities (6–35), and their growth and toxin production was monitored. Presence of saxitoxin (STX) genes (sxtA1 and sxtA4 motifs) in each strain was also analyzed. Salinity significantly affected both growth rate and toxicity of the individual strains but did not change their major toxin profile. The two Baltic Sea strains exhibited growth at salinities 6–25 and consistently produced gonyautoxin (GTX) 2, GTX3 and STX. The two North Sea strains grew at salinities 20–35 and produced mainly 20-methyl spirolide G (20mG), whereas the strain originating from the northern coast of Ireland was able to grow at salinities 15–35, only producing 13-desmethyl spirolide C (13dmC). The effects of salinity on total cellular toxin concentration and distribution of toxin analogs were strain-specific. Both saxitoxin gene motifs were present in the Baltic Sea strains, whereas the 2 North Sea strains lacked sxtA4, and the Irish strain lacked both motifs. Thus sxtA4 only seems to be specific for PST producing strains. The results show that toxin profiles of A. ostenfeldii/peruvianum strains are predetermined and the production of either spirolides or PSTs cannot be induced by salinity changes. However, changes in salinity may lead to changed growth rates, total cellular toxin concentrations as well as relative distribution of the different PST and spirolide analogs, thus affecting the actual toxicity of A. ostenfeldii/peruvianum populations.     

Survival, activity and feeding ability of Baltic cod (Gadus morhua) yolk-sac larvae at different salinities

Due to unfavourable conditions (declines in salinity and water oxygen content) in the spawning areas, there has been a considerable decrease in the Baltic cod stock since the beginning of the 1980s, and consequently a decrease in catches. In order to examine the feasibility of introducing yolk-sac larvae in areas of low salinity to improve the stock, laboratory experiments were performed on the effects of salinity on the survival, level of activity and feeding ability of larvae. Yolk-sac larvae from spawning cod caught off northern Gotland, Sweden, were exposed to four different salinities: 10 and 15%○ (salinities of the main spawning areas); and 5 and 7%○ (salinities in the Bothnian Sea and the Baltic proper respectively).
The survival of yolk-sac larvae was high at all salinities, even though there was an indication of higher mortality at low salinities in less viable larval groups. No differences were found in swimming speed or feeding ability at the four salinities, but a significant difference in vertical distribution was recorded. There were significant differences in survival, vertical distribution and feeding ability among larval groups, which indicates that larval quality or viability is of greater importance for larval survival than salinity, in the range of 5–15%○.     

Ecology and long-term forecasting of sprat (<Emphasis Type="Italic">Sprattus sprattus balticus</Emphasis>) stock in the Baltic Sea: a review

In the brackish Baltic Sea situated in the transition zone between the Atlantic and Euro-Asiatic continental climate systems, the periods of high abundance of sprat of marine boreal origin coincide with a rich freshwater discharge, large water volume inhabitable by fish, rather high winter temperature and low salinity limiting the stock of its main predator—cod (Gadus morhua callarias). In the freshening periods an additional volume of water with acceptable oxygen and temperature conditions for sprat is formed in the Eastern (E) and, especially, the Northwest (NW) regions of the Baltic Sea. This allows extraordinary increase in sprat abundance/biomass. The conditions for sprat deteriorate during the period of active saline water inflows and colder winters, especially in the NW region. Following the decrease in the volume of water acceptable for the wintering of sprat as to temperature and/or oxygen concentration, some part of the stock may be forced to migrate southwards as in the Southwest (SW) region the conditions are the most stable. This may cause an extensive mixing of the stock components of various regions, hindering the differentiation of sprat regional units in the Baltic Sea. As no isolation of regional spawning concentrations has been found, beginning with 1989 the Baltic sprat has been assessed and managed as one stock unit, despite spatial differences in its morphological characters, growth rate etc. The alternation of periods of different regime-forming conditions (freshening or oceanization) is probably triggered by climate changes. This regularity has been exploited for the composition of long-term forecasts of qualitative changes in fish (incl. sprat) stocks in the Baltic Sea.     

Warming shelf seas drive the subtropicalization of European pelagic fish communities

Pelagic fishes are among the most ecologically and economically important fish species in European seas. In principle, these pelagic fishes have potential to demonstrate rapid abundance and distribution shifts in response to climatic variability due to their high adult motility, planktonic larval stages, and low dependence on benthic habitat for food or shelter during their life histories. Here, we provide evidence of substantial climate‐driven changes to the structure of pelagic fish communities in European shelf seas. We investigated the patterns of species‐level change using catch records from 57 870 fisheries‐independent survey trawls from across European continental shelf region between 1965 and 2012. We analysed changes in the distribution and rate of occurrence of the six most common species, and observed a strong subtropicalization of the North Sea and Baltic Sea assemblages. These areas have shifted away from cold‐water assemblages typically characterized by Atlantic herring and European sprat from the 1960s to 1980s, to warmer‐water assemblages including Atlantic mackerel, Atlantic horse mackerel, European pilchard and European anchovy from the 1990s onwards. We next investigated if warming sea temperatures have forced these changes using temporally comprehensive data from the North Sea region. Our models indicated the primary driver of change in these species has been sea surface temperatures in all cases. Together, these analyses highlight how individual species responses have combined to result in a dramatic subtropicalization of the pelagic fish assemblage of the European continental shelf.     

Diversity and abundance of “Pelagibacterales” (SAR11) in the Baltic Sea salinity gradient

The candidate order “Pelagibacterales” (SAR11) is one of the most abundant bacterial orders in ocean surface waters and, periodically, in freshwater lakes. The presence of several stable phylogenetic lineages comprising “Pelagibacterales” correlates with the physico-chemical parameters in aquatic environments. A previous amplicon sequencing study covering the bacterial community in the salinity gradient of the Baltic Sea suggested that pelagibacteral subclade SAR11-I was replaced by SAR11-IIIa in the mesohaline region of the Baltic Sea. In this current study, we investigated the cellular abundances of “Pelagibacterales” subclades along the Baltic Sea salinity gradient using catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH). The results obtained with a newly designed probe, which exclusively detected SAR11-IIIa, were compared to CARD-FISH abundances of the marine SAR11-I/II subclade and the freshwater lineage SAR11-IIIb (LD12). The results showed that SAR11-IIIa was abundant in oligohaline–mesohaline conditions (salinities 2.7–13.3), with maximal abundances at a salinity of 7 (up to 35% of total Bacteria, quantified with a universal bacterial probe EUB). As expected, SAR11-I/II was abundant (27% of EUB) in the marine parts of the Baltic Sea, whereas counts of the freshwater lineage SAR11-IIIb were below the detection limit at all stations. The shift from SAR11-IIIa to SAR11-I/II was confirmed in the vertical salinity gradient in the deeper basins of the Baltic Sea. These findings were consistent with an overlapping but defined distribution of SAR11-I/II and SAR11-IIIa in the salinity gradient of the Baltic Sea and suggested the adaptation of SAR11-IIIa for growth and survival in mesohaline conditions.     

Effects of temperature and salinity on embryonic development of turbot (Scophthalmus maximus L.) from the North Sea, and comparisons with Baltic populations

Optimum temperature and salinity conditions for viable hatch were studied for turbot (Scophthalmus maximus L.) from the North Sea. Temperatures ranging from 6 to 22°C and salinities from 5 to 35‰ were used. Optimum conditions were observed to be between 12 and 18°C at salinities between 20 and 35‰. This contrasted with corresponding data for turbot from the southern Baltic proper, according to which survival sharply decreased in temperatures below 14°C and was high in salinities of 10 to 15‰. Thus, it is concluded that Baltic and Atlantic turbot should be considered as different races.     

Impact of 21st century climate change on the Baltic Sea fish community and fisheries

The Baltic Sea is a large brackish semienclosed sea whose species-poor fish community supports important commercial and recreational fisheries. Both the fish species and the fisheries are strongly affected by climate variations. These climatic effects and the underlying mechanisms are briefly reviewed. We then use recent regional – scale climate – ocean modelling results to consider how climate change during this century will affect the fish community of the Baltic and fisheries management. Expected climate changes in northern Europe will likely affect both the temperature and salinity of the Baltic, causing it to become warmer and fresher. As an estuarine ecosystem with large horizontal and vertical salinity gradients, biodiversity will be particularly sensitive to changes in salinity which can be expected as a consequence of altered precipitation patterns. Marine-tolerant species will be disadvantaged and their distributions will partially contract from the Baltic Sea; habitats of freshwater species will likely expand. Although some new species can be expected to immigrate because of an expected increase in sea temperature, only a few of these species will be able to successfully colonize the Baltic because of its low salinity. Fishing fleets which presently target marine species (e.g. cod, herring, sprat, plaice, sole) in the Baltic will likely have to relocate to more marine areas or switch to other species which tolerate decreasing salinities. Fishery management thresholds that trigger reductions in fishing quotas or fishery closures to conserve local populations (e.g. cod, salmon) will have to be reassessed as the ecological basis on which existing thresholds have been established changes, and new thresholds will have to be developed for immigrant species. The Baltic situation illustrates some of the uncertainties and complexities associated with forecasting how fish populations, communities and industries dependent on an estuarine ecosystem might respond to future climate change.     

Ontogenesis of osmotic regulation in the striped mullet, Mugil cephalus L.

The osmoregulatory capabilities of juvenile striped mullet, Mugil cephalus L ., of three size-groups (20–29, 30–39 and 40–9 mm s.l.) were compared in a series of six salinities ranging from fresh water to full sea water. The two smaller size-groups were able to tolerate instantaneous transfer from the brackish water in which captured, to all salinities but fresh water, while the 40–69 mm group were tolerant of instantaneous transfer to all experimental salinities. At high environmental salinities, the osmotic regulatory capabilities improved with growth in size to an apparently definitive condition in fish of 40–69 mm s.l. The smallest of these individuals were estimated to be about 7.5 months old. The osmotic regulatory capability in waters of low salinity had reached a definitive state with the development of tolerance to such salinities. The ontogenetic pattern for Mugil cephalus is virtually identical to that of the Atlantic salmon, Salmo salar .