Effect of acclimation temperature on the upper thermal tolerance of Colorado River cutthroat trout Oncorhynchus clarkii pleuriticus: thermal limits of a North American salmonid

In an effort to explore the thermal limitations of Colorado River cutthroat trout Oncorhynchus clarkii pleuriticus, the critical thermal maxima (T_cmax) of 1+ year Lake Nanita strain O. c. pleuriticus were evaluated when acclimated to 10, 15 and 20° C. The mean ±s.d. T_cmax for O. c. pleuriticus acclimated to 10° C was 24·6 ± 2·0°C (n = 30), for 15° C‐acclimated fish was 26·9 ± 1·5° C (n = 23) and for 20° C‐acclimated fish was 29·4 ± 1·1° C (n = 28); these results showed a marked thermal acclimation effect (Q₁₀ = 1·20). Interestingly, there was a size effect within treatments, wherein the T_cmax of larger fish was significantly lower than that of smaller fish acclimated to the same temperature. The critical thermal tolerances of age 0 year O. c. pleuriticus were also evaluated from three separate populations: Lake Nanita, Trapper Creek and Carr Creek reared under ‘common‐garden’ conditions prior to thermal acclimation. The Trapper Creek population had significantly warmer T_cmax than the Lake Nanita population, but that of the Carr Creek fish had T_cmax similar to both Trapper Creek and Lake Nanita fish. A comparison of these O. c. pleuriticus T_cmax results with those of other stream‐dwelling salmonids suggested that O. c. pleuriticus are less resistant to rapid thermal fluctuations when acclimated to cold temperatures, but can tolerate similar temperatures when acclimated to warmer temperatures.     

Rapid fluctuations in flow and water-column properties in Asan Bay, Guam: implications for selective resilience of coral reefs in warming seas

Hydrodynamics and water-column properties were investigated off west-central Guam from July 2007 through January 2008. Rapid fluctuations, on time scales of 10s of min, in currents, temperature, salinity, and acoustic backscatter were observed to occur on sub-diurnal frequencies along more than 2 km of the fore reef but not at the reef crest. During periods characterized by higher sea-surface temperatures (SSTs), weaker wind forcing, smaller ocean surface waves, and greater thermal stratification, rapid decreases in temperature and concurrent rapid increases in salinity and acoustic backscatter coincided with onshore-directed near-bed currents and offshore-directed near-surface currents. During the study, these cool-water events, on average, lasted 2.3 h and decreased the water temperature 0.57 °C, increased the salinity 0.25 PSU, and were two orders of magnitude more prevalent during the summer season than the winter. During the summer season when the average satellite-derived SST anomaly was +0.63 °C, these cooling events, on average, lowered the temperature 1.14 °C along the fore reef but only 0.11 °C along the reef crest. The rapid shifts appear to be the result of internal tidal bores pumping cooler, more saline, higher-backscatter oceanic water from depths >50 m over cross-shore distances of 100 s of m into the warmer, less saline waters at depths of 20 m and shallower. Such internal bores appear to have the potential to buffer shallow coral reefs from predicted increases in SSTs by bringing cool, offshore water to shallow coral environments. These cooling internal bores may also provide additional benefits to offset stress such as supplying food to thermally stressed corals, reducing stress due to ultraviolet radiation and/or low salinity, and delivering coral larvae from deeper reefs not impacted by surface thermal stress. Thus, the presence of internal bores might be an important factor locally in the resilience of select coral reefs facing increased thermal stress.     

Catastrophic mortality on inshore coral reefs of the Florida Keys due to severe low‐temperature stress

Coral reefs of the Florida Keys typically experience seasonal temperatures of 20–31 °C. Deviation outside this range causes physiological impairment of reef‐building corals, potentially leading to coral colony death. In January and February 2010, two closely spaced cold fronts, possibly driven by an unusually extreme Arctic Oscillation, caused sudden and severe seawater temperature declines in the Florida Keys. Inshore coral reefs [e.g., Admiral Reef (ADM)] experienced lower sustained temperatures (i.e., < 12 °C) than those further offshore [e.g., Little Grecian Reef (LG), minimum temperature = 17.2 °C]. During February and March 2010, we surveyed ADM and observed a mass die‐off of reef‐building corals, whereas 12 km away LG did not exhibit coral mortality. We subsequently measured the physiological effects of low‐temperature stress on three common reef‐building corals (i.e., Montastraea faveolata, Porites astreoides, and Siderastrea siderea) over a range of temperatures that replicated the inshore cold‐water anomaly (i.e., from 20 to 16 to 12 °C and back to 20 °C). Throughout the temperature modulations, coral respiration as well as endosymbiont gross photosynthesis and maximum quantum efficiency of photosystem II were measured. In addition, Symbiodinium genotypic identity, cell densities, and chlorophyll a content were determined at the beginning and conclusion of the experiment. All corals were significantly affected at 12 °C, but species‐specific physiological responses were found indicating different coral and/or Symbiodinium cold tolerances. Montastraea faveolata and P. astreoides appeared to be most negatively impacted because, upon return to 20 °C, significant reductions in gross photosynthesis and dark respiration persisted. Siderastrea siderea, however, readily recovered to pre‐treatment rates of dark respiration and gross photosynthesis. Visual surveys of inshore reefs corroborated these results, with S. siderea being minimally affected by the cold‐water anomaly, whereas M. faveolata and P. astreoides exhibited nearly 100% mortality. This study highlights the importance of understanding the physiological attributes of genotypically distinct coral‐Symbiodinium symbioses that contribute to tolerance, recovery, and consequences to an environmental perturbation. These data also document effects of a rarely studied environmental stressor, possibly initiated by remote global climate events, on coral‐Symbiodinium symbioses and coral reef communities.     

Investigating the behavioural responses of trapped fishes using underwater video surveillance

An underwater video surveillance system known as TrapCam was used to continuously record (15 ×c. 24 h periods) fish behaviour within and immediately surrounding an experimental fish trap situated in a coral reef ecosystem in the United States Virgin Islands. Of the 100 fishes (18 species, 12 families) trapped, surgeonfishes (Acanthuridae) and snappers (Lutjanidae) were most common. Thirteen distinctively identifiable behaviours were observed for trapped fishes. Species did not differ significantly in the proportion of time allocated to different behaviours (ANOSIM, R = 0·142). Doctorfish Acanthurus chirurgus and grey angelfish Pomacanthus arcuatus allocated the largest proportion of their recorded time to enter and exit the trap. Fishes spent an average of 15 min in the trap before escaping. Sixty‐seven per cent of trap approaches consisted of an individual of the same species as one already trapped suggesting that conspecific attraction may have occurred. Fifteen per cent of trapped species were observed with abrasions to the head and 70% were observed approaching the trap corners. The results of this study provide a greater understanding of the behavioural interactions between fishes and traps that can help explain patterns of catch composition, the physical condition of fishes in traps and inform design of gear modifications to optimize by‐catch reduction in the trap fishery.     

Variation in subsurface seawater temperature off Discovery Bay, Jamaica and the U.S. Virgin Islands

Long-term, high accuracy seawater temperature data sets are essential in studies assessing environmental changes that may alter coral reef communities. Located at the approximately the same latitude, the subsurface seawater temperature (S3T) off Discovery Bay, Jamaica (DBJ) and the U.S. Virgin Islands (USVI) had the same overall mean temperature. The USVI S3T during the winter months is approximately 0.5 degrees C warmer than DBJ, while May - July at DBJ is approximately 1 degrees C warmer than USVI S3T. With the passing of tropical storms in 1995 and 1997 in the USVI S3T dropped as much as 1.5 degrees C within a 20 hr period and did not revert to the previous temperature during that calendar year. Mean monthly S3T during 2000 and 2001 in the USVI was > 0.5 degrees C warmer than during similar periods in the early 1990s. Mean monthly S3T during 1999-2002 at DBJ was 0.27 degrees C cooler than during 1994-1995.     

Influence of sea temperature and initial marine feeding on survival of Atlantic salmon Salmo salar post-smolts from the Rivers Orkla and Hals, Norway

The abundance of returning adult Atlantic salmon Salmo salar, in the River Orkla in mid‐norway (1 sea‐winter, SW, fish) and River Hals in north Norway (1–3 SW fish), was tested against the early marine feeding and the seawater temperature experienced by their corresponding year classes of post‐smolts immediately after entry into the Trondheimsfjord (Orkla smolts, 22 years of data) and Altafjord (Hals smolts, 17 years of data). In both river–fjord systems, there was a significant positive correlation between the abundance of returning S. salar and the mean seawater temperature at the time of smolts descending to the sea. The number of 1SW fish reported caught in River Orkla was positively correlated to the proportion of fish larvae in the post‐smolt stomachs in Trondheimsfjord. The abundance of returning S.salar was, however, neither correlated to forage ratio (R_F) nor other prey groups in post‐smolt stomachs in the two fjord systems. In the Altafjord, the post‐smolts fed mainly on pelagic fish larva (70–98%) and had a stable R_F (0·009–0·023) over the 6 years analysed. In the Trondheimsfjord, however, there was a higher variation in R_F (0·003–0·036), and pelagic fish larvae were dominant prey in only two (50 and 91%) of the 8 years analysed. These 2 years also showed the highest return rates of S. salar in River Orkla. These results demonstrate that the thermal conditions experienced by post‐smolts during their early sea migration may be crucial for the subsequent return rate of adults after 1–3 years at sea. Pelagic marine fish larvae seem to be the preferred initial prey for S. salar post‐smolts. As the annual variation in abundance of fish larvae is related to seawater temperature, it is proposed that seawater temperature at sea entry and the subsequent abundance of returning adult S. salar may be indirectly linked through variation in annual availability of pelagic fish larvae or other suitable food items in the early post‐smolt phase.     

Upper thermal tolerance of wild‐type,domesticated and growth hormone‐transgenic coho salmon Oncorhynchus kisutch

In coho salmon Oncorhynchus kisutch, no significant differences in critical thermal maximum (c. 26·9° C, C_Tmax) were observed among size‐matched wild‐type, domesticated, growth hormone (GH)‐transgenic fish fed to satiation, and GH‐transgenic fish on a ration‐restricted diet. Instead, GH‐transgenic fish fed to satiation had significantly higher maximum heart rate and Arrhenius breakpoint temperature (mean ± s.e. = 17·3 ± 0·1° C, T_AB). These results provide insight into effects of modified growth rate on temperature tolerance in salmonids, and can be used to assess the potential ecological consequences of GH‐transgenic fishes should they enter natural environments with temperatures near their thermal tolerance limits.     

Aquatic food‐web dynamics following incorporation of nutrients derived from Atlantic anadromous fishes

Changes in the isotopic composition (δ¹³C and δ¹⁵N) in biofilm, macro‐invertebrates and resident salmonids were used to characterize temporal dynamics of marine derived nutrients (MDNs) incorporation between stream reaches with and without MDN inputs. Five Atlantic rivers were chosen to represent contrasting MDN subsidies: four rivers with considerable numbers of anadromous fishes; one river with little MDN input. Rainbow smelt Osmerus mordax, alewife Alosa pseudoharengus, sea lamprey Petromyzon marinus and Atlantic salmon Salmo salar, were the primary anadromous species for the sampled rivers. Regardless of the spatial resolution or the pathway of incorporation, annual nutrient pulses from spawning anadromous fishes had a positive effect on isotopic enrichment at all trophic levels (biofilm, 1·2–5·4‰; macro‐invertebrates, 0·0–6·8‰; fish, 1·2–2·6‰). Community‐wide niche space shifted toward the marine‐nutrient source, but the total ecological niche space did not always increase with MDN inputs. The time‐integrated marine‐nutrient resource contribution to the diet of S. salar parr and brook trout Salvelinus fontinalis ranged between 16·3 and 36·0% during anadromous fish‐spawning periods. The high degree of spatio‐temporal heterogeneity in marine‐nutrient subsidies from anadromous fishes lead to both direct and indirect pathways of MDN incorporation into stream food webs. This suggests that organisms at many trophic levels derive a substantial proportion of their energy from marine resources when present. The current trend of declining anadromous fish populations means fewer nutrient‐rich marine subsidies being delivered to rivers, diminishing the ability to sustain elevated riverine productivity.     

Symbiont communities and host genetic structure of the brain coral Platygyra verweyi, at the outlet of a nuclear power plant and adjacent areas

In the context of rising seawater temperatures associated with climate change, the issue of whether coral holobionts deal with this challenge by shuffling their associations with stress‐ and/or heat‐tolerant Symbiodinium, by generating heat‐resistant host genotypes, or both is important for coral survival. In this study, the composition of communities of the endosymbiont Symbiodinium and the population genetics of the coral host Platygyra verweyi were examined in a reef impacted by hot‐water discharged from the outlet of a nuclear power plant in operation in Kenting, Southern Taiwan since 1984. The water at this site is 2.0–3.0 °C warmer than adjacent reefs in summer, which have an average seawater temperature of 29.0 °C. The data were compared with those for the same species at other sites within 12 km of the outlet site. Platygyra verwyei was associated with one or both of Symbiodinium types C3 (heat sensitive) and D1a (heat tolerant) at all sites with the latter being the dominant at the nuclear power plant outlet. The proportion of C3 in populations increased gradually with increasing distance from the hot‐water discharge. Genetic analysis of the Platygyra verweyi host using mitochondrial and nuclear markers showed no genetic differentiation among sites. Changes in the composition of Symbiodinium types associated with P. verweyi among closely located sites in Kenting suggested that this coral might have acclimatized to the constant thermal stress by selective association with heat‐tolerant Symbiodinium types, whereas the role of the host in adaptation was inconclusive.     

Long‐term effects of photoperiod,temperature and their interaction on growth,gill Na+, K+‐ATPase activity,seawater tolerance and plasma growth‐hormone levels in Atlantic salmon Salmo salar

This study was undertaken to examine the long‐term effects of photoperiod, temperature and their interaction on growth, gill Na⁺,K⁺‐ATPase (NKA) activity, seawater tolerance and plasma growth‐hormone levels in Atlantic salmon Salmo salar pre‐smolts and smolts. The fish (mean ± s.e . initial body mass = 15·9 ± 0·4 g) were reared on two photoperiods (continuous light, LL, and simulated natural photoperiod, LDN, 60° 25′ N) and two temperatures (8·3 and 12·7° C) from June to May of the following year. Mean body mass was affected by photoperiod, temperature and their interactions. Both temperature groups on LL developed peak levels in gill NKA activity from October to November, 4–5 months prior to the natural season for the parr–smolt transformation. Fish at 12° C showed peak levels in NKA activity 4–6 weeks before the fish at 8° C. Fish in all four experimental groups showed maximum NKA activity within a similar size range (113–162 g). The present findings further indicate that smoltification in S. salar is to some extent driven by size, and that S. salar will develop smolt characteristics, e.g. a marked increase in NKA activity, within a similar size range. Faster‐growing S. salar will, thus, reach this size threshold at a relatively younger age.     

Temporal plasticity in thermal‐habitat selection of burbot Lota lota a diel‐migrating winter‐specialist

In this study, animal‐borne telemetry with temperature sensors was coupled with extensive habitat temperature monitoring in a dimictic reservoir, to test the following hypotheses: behavioural thermoregulation occurs throughout the year and temperature selection varies on a diel and seasonal basis, in a winter‐specialist diel‐migrating fish. Burbot Lota lota demonstrated nightly behavioural thermoregulation throughout the year, with a large seasonal shift between selection for very cold temperatures (<2° C) optimal for reproduction during the spawning period and selection for warmer temperatures (12–14° C) optimal for hunting and feeding during non‐reproductive periods. During daylight hours, while L. lota avoided habitats warmer than optimal for reproduction and feeding during the spawning and non‐reproductive periods, respectively, active selection was limited to selection for 4–6° C habitat during the prespawning period. Although behavioural thermoregulation explained the night‐time migration, behavioural thermoregulation only partially explained daytime behaviour, indicating that diel migration is best explained by a combination of factors. Thus, thermal‐habitat selection was a good predictor of night‐time habitat occupancy in a diel‐migrating species. Together, these results show that thermal‐habitat selection by fishes may be important throughout the year and a more seasonally plastic behaviour than previously recognized.     

Genomic approaches with natural fish populations

Natural populations v. inbred stocks provide a much richer resource for identifying the effects of nucleotide substitutions because natural populations have greater polymorphism. Additionally, natural populations offer an advantage over most common research organisms because they are subject to natural selection, and analyses of these adaptations can be used to identify biologically important changes. Among fishes, these analyses are enhanced by having a wide diversity of species (>28 000 species, more than any other group of vertebrates) living in a huge range of environments (from below freezing to > 46° C, in fresh water to salinities >40 ppt.). Moreover, fishes exhibit many different life‐history and reproductive strategies and have many different phenotypes and social structures. Although fishes provide numerous advantages over other vertebrate models, there is still a dearth of available genomic tools for fishes. Fishes make up approximately half of all known vertebrate species, yet <0·2% of fish species have significant genomic resources. Nonetheless, genomic approaches with fishes have provided some of the first measures of individual variation in gene expression and insights into environmental and ecological adaptations. Thus, genomic approaches with natural fish populations have the potential to revolutionize fundamental studies of diverse fish species that offer myriad ecological and evolutionary questions.     

Acclimation of Atlantic cod (Gadus morhua) to cold water: Sxtress response,osmoregulation, gill lipid composition and gill Na-K-ATPase activity

Atlantic cod (0.8–2.5 kg) were transferred froni 8°C to 1°C seawater for 17 days. No fish died. Exposure to 1°C water produced no changes in hematocrit, or in plasma concentrations of Cl⁻ or Mg²⁺. Cold water exposure caused a marked increase in plasma cortisol and glucose concentrations. Fish in 1°C seawater had higher gill Na-K-ATPase activity than fish in 8°C seawater, whereas there were no differences in gill lipid class or fatty acid composition.     

Detachment of Porites cylindrica nubbins by herbivorous fishes

A form of active restoration for coral assemblages involves culturing coral nubbins at nursery sites before transplantation to recipient reefs. Incidental grazing and/or directed predation by local fish assemblages are major sources of dislodgement and mortality for coral nubbins in nurseries. However, the rate of coral nubbin detachment, how this varies across fish taxa, and whether nubbin size affects rates of detachment warrant further investigation. We used field and aquaria experiments to examine the effect of incidental grazing and predation on the detachment of Porites cylindrica nubbins of different sizes (0.5, 1, 2, 3, 4, and 5 cm height). Short‐term (6 hours) exposure of nubbins to local fish assemblages at Lucero Reef, northwestern Philippines, caused higher detachment (1.93% ± 0.53 SE) compared to caged controls (0.16% ± 0.16 SE), with no detectable effect of nubbin size. To identify the impact of individual fish species, nubbins were exposed to one of four locally abundant herbivorous and corallivorous fish species in aquaria for 8 hours. Nubbin detachment was greater when exposed to Chlorurus spilurus (1.20–36.2%) and Siganus fuscescens (0.00–15.0%) than Chaetodon lunulatus (0.00–4.00%) and Chaetodon kleinii (0.00–1.20%), with the smallest nubbins (0.5 cm) being the most vulnerable. Our results suggest that incidental grazing by herbivorous fishes, especially parrotfishes, may potentially be an important source of detachment and likely mortality of nubbins. Optimizing coral nursery protocols should consider potential trade‐offs between excluding grazing fishes and the accumulation of algal material on caging structures to minimize nubbin mortality and improve coral restoration success.     

Novel species interactions and environmental conditions reduce foraging competency at the temperate range edge of a range-extending coral reef fish

Poleward range extensions of coral reef species can reshuffle temperate communities by generating competitive interactions that did not exist previously. However, novel environmental conditions and locally adapted native temperate species may slow tropical invasions by reducing the ability of invaders to access local resources (e.g. food and shelter). We test this hypothesis on wild marine fish in a climate warming hotspot using a field experiment encompassing artificial prey release. We evaluated seven behaviours associated with foraging and aggressive interactions in a common range-extending coral reef fish (Abudefduf vaigiensis) and a co-shoaling temperate fish (Microcanthus strigatus) along a latitudinal temperature gradient (730 km) in SE Australia. We found that the coral reef fish had reduced foraging performance (i.e. slower prey perception, slower prey inspection, decreased prey intake, increased distance to prey) in their novel temperate range than in their subtropical range. Furthermore, higher abundance of temperate fishes was associated with increased retreat behaviour by coral reef fish (i.e. withdrawal from foraging on released prey), independent of latitude. Where their ranges overlapped, temperate fish showed higher foraging and aggression than coral reef fish. Our findings suggest that lower foraging performance of tropical fish at their leading range edge is driven by the combined effect of environmental factors (e.g. lower seawater temperature and/or unfamiliarity with novel conditions in their extended temperate ranges) and biological factors (e.g. increased abundance and larger body sizes of local temperate fishes). Whilst a future increase in ocean warming is expected to alleviate current foraging limitations in coral reef fishes at leading range edges, under current warming native temperate fishes at their trailing edges appear able to slow the range extension of coral reef fishes into temperate ecosystems by limiting their access to resources.

     

Chronic stress of rainbow trout Oncorhynchus mykiss at high altitude: a field study

The stress response of Oncorhynchus mykiss in high‐altitude farms in central Mexico was investigated over two seasons: the cool (9·1–13·7° C) dry winter season, and the warmer (14·7–15·9° C), wetter summer season. Fish were subjected to an acute stress test followed by sampling of six physiological variables: blood cortisol, glucose, lactate, total antioxidant capacity, haemoglobin concentration and per cent packed cell volume (V_PC%). Multivariate analyses revealed that lactate and total antioxidant capacity were significantly higher in the summer, when water temperatures were warmer and moderate hypoxia (4·9–5·3 mg l^?1) prevailed. In contrast, plasma cortisol was significantly higher in the winter (mean ± s.e .: 76·7 ± 4·0 ng ml^?1) when temperatures were cooler and dissolved oxygen levels higher (6·05–7·9 mg l^?1), than in the summer (22·7 ± 3·8 ng ml^?1). Haemoglobin concentrations (mg dl^?1) were not significantly different between seasons, but V_PC% was significantly higher in the summer (50%) than in the winter (35%). These results suggest that in summer, effects of high altitude on farmed fish are exacerbated by stresses of high temperatures and hypoxia, resulting in higher blood lactate, increased total antioxidant capacity and elevated V_PC% levels.     

Characterization of the mesophotic reef fish community in south Florida,USA

The southeastern coast of Florida, USA supports a substantial recreational fishery, yet little is known of the coral reef ecosystem or fisheries resources past 50 m depth. Fish assemblages associated with low‐relief substrate and three vessel reefs between 50 and 120 m depth off southeast Florida were surveyed by remotely operated vehicles providing the first characterization of the mesophotic fish assemblages in the region. Two distinct assemblages were observed on low‐relief substrate and high‐relief vessel reefs. A total of 560 fishes of 42 species was recorded on 27 dives on low‐relief substrate, and 50 152 fishes of 65 species were recorded on 24 dives on three vessel reefs. Small planktivorous Anthiinae fishes and several economically valuable species were common on vessel reefs but rare on low‐relief substrate. Fish assemblages on vessel reefs more closely resembled those found at similar depths in high‐relief natural areas off east‐central Florida and the Gulf of Mexico than those associated with adjacent low‐relief habitat or nearby coral reef tracts. From a fisheries perspective, these results provide limited support to the hypothesis that in deep‐water regions with limited relief, vessel reefs may provide an opportunity to increase fish diversity and abundance by creating high‐relief habitat without compromising adjacent fish assemblages.     

Why are there so few freshwater fish species in most estuaries?

The freshwater fish assemblage in most estuaries is not as species rich as the marine assemblage in the same systems. Coupled with this differential richness is an apparent inability by most freshwater fish species to penetrate estuarine zones that are mesohaline (salinity: 5·0–17·9), polyhaline (salinity: 18·0–29·9) or euhaline (salinity: 30·0–39·9). The reason why mesohaline waters are avoided by most freshwater fishes is difficult to explain from a physiological perspective as many of these species would be isosmotic within this salinity range. Perhaps, a key to the poor penetration of estuarine waters by freshwater taxa is an inability to develop chloride cells in gill filament epithelia, as well as a lack of other osmoregulatory adaptations present in euryhaline fishes. Only a few freshwater fish species, especially some of those belonging to the family Cichlidae, have become fully euryhaline and have successfully occupied a wide range of estuaries, sometimes even dominating in hyperhaline systems (salinity 40+). Indeed, this review found that there are few fish species that can be termed holohaline (i.e. capable of occupying waters with a salinity range of 0–100+) and, of these taxa, there is a disproportionally high number of freshwater species (e.g. Cyprinodon variegatus, Oreochromis mossambicus and Sarotherodon melanotheron). Factors such as increased competition for food and higher predation rates by piscivorous fishes and birds may also play an important role in the low species richness and abundance of freshwater taxa in estuaries. Added to this is the relatively low species richness of freshwater fishes in river catchments when compared with the normally higher diversity of marine fish species for potential estuarine colonization from the adjacent coastal waters. The almost complete absence of freshwater fish larvae from the estuarine ichthyoplankton further reinforces the poor representation of this guild within these systems. An explanation as to why more freshwater fish species have not become euryhaline and occupied a wide range of estuaries similar to their marine counterparts is probably due to a combination of the above described factors, with physiological restrictions pertaining to limited salinity tolerances probably playing the most important role.