Divergent responses of Atlantic cod to ocean acidification and food limitation

In order to understand the effect of global change on marine fishes, it is imperative to quantify the effects on fundamental parameters such as survival and growth. Larval survival and recruitment of the Atlantic cod (Gadus morhua) were found to be heavily impaired by end‐of‐century levels of ocean acidification. Here, we analysed larval growth among 35–36 days old surviving larvae, along with organ development and ossification of the skeleton. We combined CO₂ treatments (ambient: 503 µatm, elevated: 1,179 µatm) with food availability in order to evaluate the effect of energy limitation in addition to the ocean acidification stressor. As expected, larval size (as a proxy for growth) and skeletogenesis were positively affected by high food availability. We found significant interactions between acidification and food availability. Larvae fed ad libitum showed little difference in growth and skeletogenesis due to the CO₂ treatment. Larvae under energy limitation were significantly larger and had further developed skeletal structures in the elevated CO₂ treatment compared to the ambient CO₂ treatment. However, the elevated CO₂ group revealed impairments in critically important organs, such as the liver, and had comparatively smaller functional gills indicating a mismatch between size and function. It is therefore likely that individual larvae that had survived acidification treatments will suffer from impairments later during ontogeny. Our study highlights important allocation trade‐off between growth and organ development, which is critically important to interpret acidification effects on early life stages of fish.     

Effect of ocean acidification on growth and otolith condition of juvenile scup,Stenotomus chrysops

Increasing amounts of atmospheric carbon dioxide (CO₂) from human industrial activities are causing changes in global ocean carbonate chemistry, resulting in a reduction in pH, a process termed “ocean acidification.” It is important to determine which species are sensitive to elevated levels of CO₂ because of potential impacts to ecosystems, marine resources, biodiversity, food webs, populations, and effects on economies. Previous studies with marine fish have documented that exposure to elevated levels of CO₂ caused increased growth and larger otoliths in some species. This study was conducted to determine whether the elevated partial pressure of CO₂ (pCO₂) would have an effect on growth, otolith (ear bone) condition, survival, or the skeleton of juvenile scup, Stenotomus chrysops, a species that supports both important commercial and recreational fisheries. Elevated levels of pCO₂ (1200–2600 μatm) had no statistically significant effect on growth, survival, or otolith condition after 8 weeks of rearing. Field data show that in Long Island Sound, where scup spawn, in situ levels of pCO₂ are already at levels ranging from 689 to 1828 μatm due to primary productivity, microbial activity, and anthropogenic inputs. These results demonstrate that ocean acidification is not likely to cause adverse effects on the growth and survivability of every species of marine fish. X‐ray analysis of the fish revealed a slightly higher incidence of hyperossification in the vertebrae of a few scup from the highest treatments compared to fish from the control treatments. Our results show that juvenile scup are tolerant to increases in seawater pCO_2, possibly due to conditions this species encounters in their naturally variable environment and their well‐developed pH control mechanisms.     

An assessment of de-calcification procedures for δ13C and δ15N analysis of yellow perch, walleye and Atlantic salmon scales

The scales of three species of fishes, yellow perch Perca flavescens , walleye Sander vitreus and Atlantic salmon Salmo salar , were acidified and the isotopic signatures were compared to non-acidified scales from the same fishes. No significant acidification effects on either carbon or nitrogen isotope signatures were found. Results contrast with earlier literature findings noting significant acidification effects and suggest acidification tests be undertaken before scales are used for temporal reconstruction of fish food web positions.     

Ocean acidification affects prey detection by a predatory reef fish

Changes in olfactory-mediated behaviour caused by elevated CO(2) levels in the ocean could affect recruitment to reef fish populations because larval fish become more vulnerable to predation. However, it is currently unclear how elevated CO(2) will impact the other key part of the predator-prey interaction--the predators. We investigated the effects of elevated CO(2) and reduced pH on olfactory preferences, activity levels and feeding behaviour of a common coral reef meso-predator, the brown dottyback (Pseudochromis fuscus). Predators were exposed to either current-day CO(2) levels or one of two elevated CO(2) levels (～600 μatm or ～950 μatm) that may occur by 2100 according to climate change predictions. Exposure to elevated CO(2) and reduced pH caused a shift from preference to avoidance of the smell of injured prey, with CO(2) treated predators spending approximately 20% less time in a water stream containing prey odour compared with controls. Furthermore, activity levels of fish was higher in the high CO(2) treatment and feeding activity was lower for fish in the mid CO(2) treatment; indicating that future conditions may potentially reduce the ability of the fish to respond rapidly to fluctuations in food availability. Elevated activity levels of predators in the high CO(2) treatment, however, may compensate for reduced olfactory ability, as greater movement facilitated visual detection of food. Our findings show that, at least for the species tested to date, both parties in the predator-prey relationship may be affected by ocean acidification. Although impairment of olfactory-mediated behaviour of predators might reduce the risk of predation for larval fishes, the magnitude of the observed effects of elevated CO(2) acidification appear to be more dramatic for prey compared to predators. Thus, it is unlikely that the altered behaviour of predators is sufficient to fully compensate for the effects of ocean acidification on prey mortality.     

Whitefish stocking in acidified lakes: ecological and physiological responses

In autumn 1986, six small lakes at different stages of acidification were stocked with one-summer-old whitefish, Coregonus pallasi Valenciennes 1848, in order to see whether whitefish stocking would be a suitable method for the mitigation of acidification effects. In two of the lakes the introduction was a complete failure: the whitefish did not survive, evidently due to high acidity and high aluminium concentrations of the lake waters. In one of the most acidified lakes (pH 4.3–4.8, Al_lab 29–125 g 1^–1) and in two less acidic lakes (pH 5.0–5.2 and 5.4–6.4), introduction was successful. Three years after the introduction, the mean weights of the fish in those three lakes were 580, 250 and 360 g respectively, with the weight and also the condition factor of stocked whitefish being highest in the most acidified lake. In that lake there were few or no fish present during the introduction, whereas in the less acid lakes there were dense populations of perch and therefore a potential interspecific competition for food. Different availability of food in the lakes was presumed to be the main reason for the growth differences. Plasma Na⁺ and Cl^– concentrations of whitefish were lower in the acidic lakes than in the lake with pH around 6 three years after stocking. This suggests that, despite the good growth and highest condition factor of whitefish in the most acid lake, the fish still experienced some acid stress.     

Effects of predation environment and food availability on somatic growth in the Livebearing Fish Brachyrhaphis rhabdophora (Pisces: Poeciliidae)

Variation in somatic growth rates is of great interest to biologists because of the relationship between growth and other fitness‐determining traits, and it results from both genetic and environmentally induced variation (i.e. plasticity). Theoretical predictions suggest that mean somatic growth rates and the shape of the reaction norm for growth can be influenced by variation in predator‐induced mortality rates. Few studies have focused on variation in reaction norms for growth in response to resource availability between high‐predation and low‐predation environments. We used juvenile Brachyrhaphis rhabdophora from high‐predation and low‐predation environments to test for variation in mean growth rates and for variation in reaction norms for growth at two levels of food availability in a common‐environment experiment. To test for variation in growth rates in the field, we compared somatic growth rates in juveniles in high‐predation and low‐predation environments. In the common‐environment experiment, mean growth rates did not differ between fish from differing predation environments, but the interaction between predation environment and food level took the form of a crossing reaction norm for both growth in length and mass. Fish from low‐predation environments exhibited no significant difference in growth rate between high and low food treatments. In contrast, fish from high‐predation environments exhibited variation in growth rates between high and low food treatments, with higher food availability resulting in higher growth rates. In the field, individuals in the high‐predation environment grow at a faster rate than those in low‐predation environments at the smallest sizes (comparable to sizes in the common‐environment experiment). These data provide no evidence for evolved differences in mean growth rates between predation environments. However, fish from high‐predation environments exhibited greater plasticity in growth rates in response to resource availability suggesting that predation environments may exhibit increased variation in food availability for prey fish and consequent selection for plasticity.     

Growth and population size of crayfish in headwater streams: individual- and higher-level consequences of acidification

1. Environmental stress may have indirect positive effects on population size through modification of food‐web interactions, despite having negative effects on individuals. Here we evaluate the individual‐ and population‐level effects of acidification on crayfish (Cambarus bartonii) in headwater streams of the Allegheny Plateau (PA, U.S.A.) with field experiments and survey data. Median baseflow pH of 24 study reaches in nine streams varied from 4.4 to 7.4, with substantial variation found both among and within streams. 2. Two bioassays were conducted to evaluate the relationship between stream pH and crayfish growth rates. Growth rates were always higher in circumneutral reaches than in acidic reaches. Crayfish originating in acidic water grew less when transplanted into neutral water than did crayfish originating in neutral water, providing some evidence for a cost of acclimation to acidity. 3. Stream surveys showed that fish were less abundant and crayfish more abundant in acidified streams than in circumneutral streams. Crayfish density was sixfold higher in reaches with the lowest pH relative to circumneutral reaches. Large crayfish made up a higher proportion of crayfish populations at sites with high fish biomass, consistent with the hypothesis that fish predation on small individuals may be limiting crayfish population size at these sites. 4. Although individual crayfish suffered lower growth in acidified streams, increased acidity appeared to cause an increase in crayfish population size and shifts in size structure, possibly by relieving predation pressure by fish.     

Acid rain recovery may help to mitigate the impacts of climate change on thermally sensitive fish in lakes across eastern North America

From the 1970s to 1990s, more stringent air quality regulations were implemented across North America and Europe to reduce chemical emissions that contribute to acid rain. Surface water pH slowly increased during the following decades, but biological recovery lagged behind chemical recovery. Fortunately, this situation is changing. In the past few years, northeastern US fish populations have begun to recover in lakes that were historically incapable of sustaining wild fish due to acidic conditions. As lake ecosystems across the eastern United States recover from acid deposition, the stress to the most susceptible populations of native coldwater fish appears to be shifting from acidification effects to thermal impacts associated with changing climate. Extreme summer temperature events – which are expected to occur with increasing frequency in the coming century – can stress and ultimately kill native coldwater fish in lakes where thermal stratification is absent or highly limited. Based on data from northeastern North America, we argue that recovery from acid deposition has the potential to improve the resilience of coldwater fish populations in some lakes to impacts of climate change. This will occur as the amount of dissolved organic carbon (DOC) in the water increases with increasing lake pH. Increased DOC will reduce water clarity and lead to shallower and more persistent lake thermoclines that can provide larger areas of coldwater thermal refuge habitat. Recovery from acidification will not eliminate the threat of climate change to coldwater fish, but secondary effects of acid recovery may improve the resistance of coldwater fish populations in lakes to the effects of elevated summer temperatures in historically acidified ecosystems. This analysis highlights the importance of considering the legacy of past ecosystem impacts and how recovery or persistence of those effects may interact with climate change impacts on biota in the coming decades.     

Are Chaoborus larvae more abundant in acidified than in non-acidified lakes in Central Canada?

Eriksson et al (1980) hypothesized that the abundance of certain macroinvertebrate predators, such as larvae of the phantom midge Chaoborus , should increase in acidified lakes because of the elimination of fish. To examine the influence of pH and presence of fish on Chaoborus abundance, we surveyed Chaoborus populations in 33 lakes in Ontario, Canada which ranged in pH from 4.5 to 7.4. Chaoborus larvae were not more abundant in the acidified lakes that were devoid of fish than in the remaining lakes. Therefore, we concluded that pH and presence of fish are not prime determinants of total Chaoborus abundance in Canadian Shield lakes. We hypothesized that significant increases in Chaoborus abundance should only be anticipated when fish populations are eliminated by acidification of relatively nutrient rich lakes.     

Impacts of invasive fish removal through angling on population characteristics and juvenile growth rate

Exploitation can modify the characteristics of fish populations through the selective harvesting of individuals, with this potentially leading to rapid ecological and evolutionary changes. Despite the well‐known effects of invasive fishes on aquatic ecosystems generally, the potential effects of their selective removal through angling, a strategy commonly used to manage invasive fish, are poorly understood. The aim of this field‐based study was to use the North American pumpkinseed Lepomis gibbosus as the model species to investigate the consequences of selective removal on their population characteristics and juvenile growth rates across 10 populations in artificial lakes in southern France. We found that the maximal individual mass in populations decreased as removal pressure through angling increased, whereas we did not observed any changes in the maximal individual length in populations as removal pressure increased. Total population abundance did not decrease as removal pressure increased; instead, here was a U‐shaped relationship between removal pressure and the abundance of medium‐bodied individuals. In addition, population biomass had a U‐shaped curve response to removal pressure, implying that invasive fish populations can modulate their characteristics to compensate for the negative effects of selective removals. In addition, individual lengths at age 2 and juvenile growth rates decreased as removal pressure through angling increased, suggesting a shift toward an earlier size at maturity and an overall slower growing phenotype. Therefore, these outputs challenge the efficiency of selective management methods, suggesting the use of more proactive strategies to control invasive populations, and the need to investigate the potential ecological and evolutionary repercussions of nonrandom removal.     

Increased CO2 stimulates reproduction in a coral reef fish

Ocean acidification is predicted to negatively impact the reproduction of many marine species, either by reducing fertilization success or diverting energy from reproductive effort. While recent studies have demonstrated how ocean acidification will affect larval and juvenile fishes, little is known about how increasing partial pressure of carbon dioxide (pCO₂) and decreasing pH might affect reproduction in adult fishes. We investigated the effects of near‐future levels of pCO₂ on the reproductive performance of the cinnamon anemonefish, Amphiprion melanopus, from the Great Barrier Reef, Australia. Breeding pairs were held under three CO₂ treatments [Current‐day Control (430 μatm), Moderate (584 μatm) and High (1032 μatm)] for a 9‐month period that included the summer breeding season. Unexpectedly, increased CO₂ dramatically stimulated breeding activity in this species of fish. Over twice as many pairs bred in the Moderate (67% of pairs) and High (55%) compared to the Control (27%) CO₂ treatment. Pairs in the High CO₂ group produced double the number of clutches per pair and 67% more eggs per clutch compared to the Moderate and Control groups. As a result, reproductive output in the High group was 82% higher than that in the Control group and 50% higher than that in the Moderate group. Despite the increase in reproductive activity, there was no difference in adult body condition among the three treatment groups. There was no significant difference in hatchling length between the treatment groups, but larvae from the High CO₂ group had smaller yolks than Controls. This study provides the first evidence of the potential effects of ocean acidification on key reproductive attributes of marine fishes and, contrary to expectations, demonstrates an initially stimulatory (hormetic) effect in response to increased pCO₂. However, any long‐term consequences of increased reproductive effort on individuals or populations remain to be determined.     

How many fish populations in Finland are affected by acid precipitation?

Synopsis The number of fish populations affected or lost from small lakes in southern and central Finland due to acid precipitation is estimated. Tolerance limits (pH and labile aluminum) of common fish species were obtained from a fish status survey of 80 lakes. These tolerance values were used to estimate the proportion of affected lakes from the water chemistry data of 783 statistically selected lakes. The proportion of anthropogenically acidified lakes was estimated by calculating pre-acidification pH and aluminum concentrations of the lakes, using a steady-state model based on water chemistry. The number of fish populations for which acidification has affected growth or population structure was estimated at between 2200 and 4400. Out of these, the number of fish populations that have disappeared due to acid precipitation would be about 1000–2000. Almost 60% of the affected or lost populations are roach, Rutilus rutilus, the most sensitive of the common fish species in small lakes in southern and central Finland. Less than 15% of the damaged population is European perch, Perca fluviatilis, the most common species. This is due to the substantially higher tolerance of perch to acidified water in comparison with roach.     

海洋酸化效应对海水鱼类的综合影响评述

人类活动引起的大气CO2浓度的升高,除了使全球温度升高外,导致的另一个严重生态问题——海洋酸化(Ocean acidification,OA),受到社会各界包括科研界的高度重视,该领域的大部分研究结果都是在近十年才发表出来的,目前还有很多需要解决的问题。海洋酸化的研究涉及到很多学科的交叉包括化学、古生物学、生态学、生物地球化学等等。在生物学领域,海洋酸化主要围绕敏感物种,例如由碳酸钙形成贝壳或外骨骼的贝类,珊瑚礁群体等。鱼类作为海洋脊椎动物的代表生物类群,自身具有一定的酸碱平衡调节能力,但相关海洋酸化方向的研究并不是很多。尽管人们对于海洋酸化对鱼类的影响了解甚少,这并不说明海洋酸化对鱼类没有作用或者效应小,在相关研究逐步展开的同时,发现鱼类同样受到海洋酸化的危害,几乎涉及到鱼类整个生活史和几乎大部分生理过程,尤其是早期生活史的高度敏感。因此就目前国内外对此领域研究结果做综述,以期待业界同行能够对海水鱼类这个大的类群引起重视。     

Soil acidification reduces the effects of short‐term nutrient enrichment on plant and soil biota and their interactions in grasslands

Soil nitrogen (N) and phosphorus (P) contents, and soil acidification have greatly increased in grassland ecosystems due to increased industrial and agricultural activities. As major environmental and economic concerns worldwide, nutrient enrichment and soil acidification can lead to substantial changes in the diversity and structure of plant and soil communities. Although the separate effects of N and P enrichment on soil food webs have been assessed across different ecosystems, the combined effects of N and P enrichment on multiple trophic levels in soil food webs have not been studied in semiarid grasslands experiencing soil acidification. Here we conducted a short‐term N and P enrichment experiment in non‐acidified and acidified soil in a semiarid grassland on the Mongolian Plateau. We found that net primary productivity was not affected by N or P enrichment alone in either non‐acidified or acidified soil, but was increased by combined N and P enrichment in both non‐acidified and acidified soil. Nutrient enrichment decreased the biomass of most microbial groups in non‐acidified soil (the decrease tended to be greatest with combined N and P enrichment) but not in acidified soil, and did not affect most soil nematode variables in non‐acidified or acidified soil. Nutrient enrichment also changed plant and microbial community structure in non‐acidified but not in acidified soil, and had no effect on nematode community structure in non‐acidified or acidified soil. These results indicate that the responses to short‐term nutrient enrichment were weaker for higher trophic groups (nematodes) than for lower trophic groups (microorganisms) and primary producers (plants). The findings increase our understanding of the effects of nutrient enrichment on multiple trophic levels of soil food webs, and highlight that soil acidification, as an anthropogenic stressor, reduced the responses of plants and soil food webs to nutrient enrichment and weakened plant–soil interactions.     

Trophic structure of coastal freshwater stream fishes from an Atlantic rainforest: evidence of the importance of protected and forest-covered areas to fish diet

The role of riparian forests in the functioning of aquatic ecosystems is well known, and they are recognized as an important food source for riverine fauna. This study investigates the trophic structure of coastal freshwater stream fishes from a large conservation area in an Atlantic rainforest using stomach content and food availability analyses. Four samples were collected from 19 sample sites. Fishes were caught with electrofishing. Prey were sampled with trays, Surber, traps, and electrofishing to evaluate the availability of food resources. The diets of 20 fish species were determined from the stomach contents of 1691 individuals. Terrestrial and aquatic insects and detritus were the most consumed items. Fish diet and prey availability were not seasonally dependent. A cluster analysis showed five trophic functional groups: terrestrial insectivores, aquatic insectivores, detritivores, carnivores, and omnivores. Insectivores predominated in species richness (60%), abundance (47%) and biomass (39%). Allochthonous and autochthonous items were found in similar proportions in the environment; however, allochthonous items were representative for insectivores and detritivores, whereas autochthonous items were important for primarily aquatic insectivores. The preference for certain insects by insectivorous fishes was associated with food selectivity rather than the availability of the resource and demonstrated the strong relationship between feeding behavior and food preference. The absence of seasonal variation in the diets of the fishes was possibly related to the consistent food supply. Our results confirm the role of the forest as a food provider for stream fishes, such as terrestrial insects and plant debris/detritus (also consumed by aquatic insects, which subsequently serve as food for fish), highlighting the importance of conserving the Brazilian Atlantic rainforests.     

Detrimental effects of ocean acidification on the economically important Mediterranean red coral (Corallium rubrum)

The mean predicted decrease of 0.3–0.4 pH units in the global surface ocean by the end of the century has prompted urgent research to assess the potential effects of ocean acidification on the marine environment, with strong emphasis on calcifying organisms. Among them, the Mediterranean red coral (Corallium rubrum) is expected to be particularly susceptible to acidification effects, due to the elevated solubility of its Mg‐calcite skeleton. This, together with the large overexploitation of this species, depicts a bleak future for this organism over the next decades. In this study, we evaluated the effects of low pH on the species from aquaria experiments. Several colonies of C. rubrum were long‐term maintained for 314 days in aquaria at two different pH levels (8.10 and 7.81, pH_T). Calcification rate, spicule morphology, major biochemical constituents (protein, carbohydrates and lipids) and fatty acids composition were measured periodically. Exposure to lower pH conditions caused a significant decrease in the skeletal growth rate in comparison with the control treatment. Similarly, the spicule morphology clearly differed between both treatments at the end of the experiment, with aberrant shapes being observed only under the acidified conditions. On the other hand, while total organic matter was significantly higher under low pH conditions, no significant differences were detected between treatments regarding total carbohydrate, lipid, protein and fatty acid composition. However, the lower variability found among samples maintained in acidified conditions relative to controls, suggests a possible effect of pH decrease on the metabolism of the colonies. Our results show, for the first time, evidence of detrimental ocean acidification effects on this valuable and endangered coral species.     

Intraspecific variation in the growth and survival of juvenile fish exposed to Eucalyptus leachate

Whilst changes in freshwater assemblages along gradients of environmental stress have been relatively well studied, we know far less about intraspecific variation to these same stressors. A stressor common in fresh waters worldwide is leachates from terrestrial plants. Leachates alter the physiochemical environment of fresh waters by lowering pH and dissolved oxygen and also releasing toxic compounds such as polyphenols and tannins, all of which can be detrimental to aquatic organisms. We investigated how chronic exposure to Eucalyptus leaf leachate affected the growth and survival of juvenile southern pygmy perch (Nannoperca australis) collected from three populations with different litter inputs, hydrology and observed leachate concentrations. Chronic exposure to elevated leachate levels negatively impacted growth and survival, but the magnitude of these lethal and sublethal responses was conditional on body size and source population. Bigger fish had increased survival at high leachate levels but overall slower growth rates. Body size also varied among populations and fish from the population exposed to the lowest natural leachate concentrations had the highest average stress tolerance. Significant intraspecific variation in both growth and survival caused by Eucalyptus leachate exposure indicates that the magnitude (but not direction) of these stress responses varies across the landscape. This raises the potential for leachate‐induced selection to operate at an among‐population scale. The importance of body size demonstrates that the timing of leachate exposure during ontogeny is central in determining the magnitude of biological response, with early life stages being most vulnerable. Overall, we demonstrate that Eucalyptus leachates are prevalent and potent selective agents that can trigger important sublethal impacts, beyond those associated with more familiar fish kills, and reiterate that dissolved organic carbon is more than just an energy source in aquatic environments.     

Reef-based micropredators reduce the growth of post-settlement damselfish in captivity

Despite the ubiquity of micropredators and parasites on coral reefs, their effects on the survival and growth of juvenile fishes are virtually unstudied. Caging and laboratory experiments were used to investigate whether reef based micropredators fed on recently metamorphosed damselfish, the time of day that micropredation occurred, and whether micropredation affected fish growth and survival. Caged juveniles of the damselfish, Pomacentrus moluccensis, were held on the reef over four consecutive time periods. Micropredators (gnathiid and cirolanid isopods) were found associated with caged fish at night only, and cirolanids were observed attacking and killing some caged fish. In order to test the effect of micropredation on growth and survival without the influence of predatory fishes, groups of five P. moluccensis were caged for 2 weeks in one of three treatments: micropredators excluded, mesh control, or micropredators present. There were no significant differences in survival among the treatments, but fish were larger in cages with fewer survivors suggesting that competition for food was intense. Fish exposed to micropredators were larger than fish in the other two treatments, however, micropredator exclusion also excluded plankton; thus, differences in food availability among treatments during crepuscular periods likely confounded the treatment effect on fish growth. A laboratory growth experiment was performed to better control food availability and minimise handling stress, using a validated host-micropredator model. Individual juvenile damselfish, Dischistodus perspicillatus, were exposed to 0, 1 or 2 micropredators (Gnathia falcipenis) each evening and fed equally for 8 days. Mortalities only occurred in fish exposed to micropredators on the first evening of the experiment, and fish exposed to two micropredators each evening were significantly smaller than unexposed fish. These results suggest that repeated gnathiid infections can reduce fish growth in the first week after settlement. Consequently, micropredation may affect the ecology of damselfish after settling on coral reefs.     

The cost of living for freshwater fish in a warmer, more polluted world

Little of the vast literature on the temperature physiology of freshwater fish is useful in predicting the effects of global warming. In the present review a series of laboratory experiments is reviewed in which rainbow trout (Oncorhynchus mykiss) were exposed to simulated global warming, a 2 °C increment superimposed upon the natural thermal regime, in the presence and absence of two common freshwater pollutants, ammonia and acidity (low pH). Simulated global warming had little effect on the growth and physiology of trout fed to satiation over much of the summer. However, in late summer, when ambient water temperature was at its highest, the addition of 2 °C caused a marked inhibition of appetite and growth, although this impact was not exacerbated by a reduction in food availability. In winter, + 2 °C stimulated metabolism, appetite and growth by approximately 30–60%. Exposure of satiation‐fed trout to low levels of pollutants produced unexpected results. Ammonia (NH₃ + NH₄⁺ = 70 μm) stimulated summer growth and energy conversion efficiency, whilst acidification (pH 5.2) increased appetite and growth but caused no disturbance of electrolyte balance. These pollutant effects were additive upon, but not synergistic with, the effects of + 2 °C. The ability of the fish to acclimate to the experimental conditions was tested with acute lethal temperature and/or toxicant challenges. Fish exposed to + 2 °C had a slightly (0.2–1.0 °C) but significantly higher lethal temperature than those exposed to ambient temperature when fed to satiation. However, there was no evidence of acclimation to either ammonia or low pH. It is concluded that the impact of global warming on freshwater fish will vary seasonally. The additional temperature may provide growth benefits in winter, but may threaten fish populations living towards the upper end of their thermal tolerance zone in (late) summer.     

Ecosystem consequences of fish parasites*

In most aquatic ecosystems, fishes are hosts to parasites and, sometimes, these parasites can affect fish biology. Some of the most dramatic cases occur when fishes are intermediate hosts for larval parasites. For example, fishes in southern California estuaries are host to many parasites. The most common of these parasites, Euhaplorchis californiensis, infects the brain of the killifish Fundulus parvipinnis and alters its behaviour, making the fish 10–30 times more susceptible to predation by the birds that serve as its definitive host. Parasites like E. californiensis are embedded in food webs because they require trophic transmission. In the Carpinteria Salt Marsh estuarine food web, parasites dominate the links and comprise substantial amount of biomass. Adding parasites to food webs alters important network statistics such as connectance and nestedness. Furthermore, some free‐living stages of parasites are food items for free‐living species. For instance, fishes feed on trematode cercariae. Being embedded in food webs makes parasites sensitive to changes in the environment. In particular, fishing and environmental disturbance, by reducing fish populations, may reduce parasite populations. Indirect evidence suggests a decrease in parasites in commercially fished species over the past three decades. In addition, environmental degradation can affect fish parasites. For these reasons, parasites in fishes may serve as indicators of environmental impacts.