Chronic effects of low pH on length and weight of Atlantic salmon,Salmo salar

Synopsis Measurements were made on Atlantic salmon from five sources in New England. Fish were reared at pH 7.0 and 5.2 from hatch to 199, 270 or 440 days. Length and weight were smaller in Atlantic salmon, Salmo salar, that were continuously exposed to low pH in the laboratory. Length and weight in neutral pH water varied among fish from the different sources. Fish from the Green Lake, Craig Brook, and Tunison sources were not significantly different in their lengths over time. Cortland and landlocked Atlantic salmon were not significantly different from one another but were significantly different from the other three sources. At pH 5.2, there were no significant differences in length between fish from any of the five sources. The Cortland salmon were significantly different from the other four sources in respect to their weight over time at neutral pH. At pH 5.2, there were no significant differences between any of the five sources.Cooperators are the U.S. Fish and Wildlife Service, the Maine Department of Inland Fisheries and Wildlife, the University of Maine, and the Wildlife Management Institute     

Seasonal absence of fishes in tidepools of a boreal environment (Maine,USA)

Tidepool fishes of a rocky, boreal community in Maine (USA) were sampled periodically from September 1979 to September 1988. Unlike tidepool fish communities in many other areas of the world, those in Maine included no year-round residents. Several of the 22 species among the 1,135 fishes collected occupied tidepools seasonally as nursery areas. In particular, juvenile lumpfish,Cyclopterus lumpus, used tidepools from June to December during their first two years of life. Disappearance of these species from tidepools in winter appeared to be primarily a result of low water temperatures and reduced algal cover in pools.Cooperators are the University of Maine, Maine Department of Inland Fisheries and Wildlife, U.S. Fish and Wildlife Service, and Wildlife Management Institute.     

Ecological model of interactions between escaped and wild Atlantic salmon Salmo salar

Atlantic salmon in Maine were once abundant but have become depleted, and are listed as endangered under the federal Endangered Species Act. Historically, salmon numbers in Maine may have been as high as 100 000 adults, but habitat loss, pollution and overfishing have contributed to the decline of the species. In 2000, only 110 adults returned to spawn in Maine rivers. Maine produces c. 15 000 metric tons/year of aquacultured Atlantic salmon from a total of nearly 600 coastal net pens. Escapees from these pens may interact with the wild salmon. The dynamics of salmon populations under such conditions are poorly understood. In order to illuminate the role aquaculture may play in such a system, we have developed a model for simulating population trajectories for both wild salmon and competing populations derived from aquaculture escapes. The model simulates a small population of wild salmon based in a stream/estuary system, into which an aquaculture facility is losing fish to escapes. Biological parameters in the model were estimated as much as possible from data in the USFWS report on Maine salmon. We used the model to investigate the consequences of a variety of ecological interactions between the wild and cultured fish including competitive, genetic and disease effects. Initial results indicate that many of these effects allow the aquaculture‐derived population to supplant the wild fish, but that wild populations may still persist under some conditions.     

Seasonal fluctuation of zooplankton populations in lower Delaware Bay

Quantitative zooplankton samples were obtained monthly or bi-monthly 15 times from June 1974 to May 1975 at three stations in lower Delaware Bay. Two 12-hour cruises were also conducted at one of the stations.Arthropods dominated the samples in terms of number of species and number of individuals. The number of zooplankton from surface samples ranged from 58/m³ in August to 21,092/ m³ in June, while bottom samples varied from 259/m³ in August to 30,395/ m³ in October. In general, larger concentrations of individuals were found in bottom samples.Only on three occasions did meroplankton exceed the holoplankton, and these occurred at the shallow water stations. Meroplankton comprised a larger percentage of the bottom samples than surface samples. Zoeae of Neopanope sayi and Uca sp. contributed mainly to the large proportion of meroplankton in July 1974, veligers of Mytilus edulis in January 1975, and nauplii of Balanus sp. in May 1975.Copepods were the largest component of the population throughout most of the year. At all stations and depths, Arctica tonsa dominated most of the summer samples. In the spring of 1975, A. tonsa was replaced by Centropages hamatus, Temora longicornis, and Pseudocalanus minutus.During the 12-hour cruises there were higher numbers of individuals in the bottom waters in the day with migration to surface waters in the afternoon and evening. Based on cluster analysis, five time-related assemblages were discerned: June, July–August, September–November, December, January–May. Comparison of Delaware Bay zooplankton with other estuarine systems indicated that the densities obtained locally were most similar to those reported in the York River, Virginia.     

Did chironomid emergence release Daphnia from fish predation and lead to a Daphnia-driven clear-water phase in Lake Towada,Japan?

The emergence of the chironomid pupae into the water column may potentially allow Daphnia to be released from positively size-selective fish predation and indirectly cause a Daphnia-driven clear-water phase in lakes. We studied these processes in Lake Towada, Japan, from April to October 1998. The biomass of the Daphnia longispina population was stable from April through June and increased by 3-fold in mid-July. This coincided well with the onset of the clear-water phase in the lake. The sharp decline of chironomid larvae in the sediment in June suggested that the major emergence occurred. Stomach contents of kokanee (Oncorhynchus nerka) and pond smelt (Hypomesus transpacificus nipponensis) also shifted from D. longispina to the chironomid pupae at that time. However, mean individual body size, mean ovigerous female size and clutch size of D. longispina declined rapidly in mid-June and stayed smaller thereafter compared to those in April–May, suggesting that fish predation was stronger after mid-June. It is probable that lower water temperatures in April–May suppressed fish feeding activity as well as reproduction in D. longispina, which resulted in the stable biomass of D. longispina. The increased water temperatures resulted in the stable biomass of D. longispina. This would lead to a mid-July peak of less predation-sensitive young adults. Therefore, the chironomid emergence seemed neither to release D. longispinafrom fish predation nor to be associated with the clear-water phase in the present study.     

Distribution patterns and predation risk of the coexisting cladocerans Bythotrephes longimanus and Leptodora kindtii in a large lake – Lake Constance

The distributions of the coexisting, planktonic, predatory cladocerans Bythotrephes longimanus and Leptodora kindtii were studied at four different stations (Langenargen-Arbon, Fischbach-Uttwil, Bregenzer Bucht and Zeller See) of a great lake, Lake Constance. The stations Langenargen-Arbon and Fischbach-Uttwil are deep (>200 m) with a high density of the coregonid fish Coregonus lavaretus, the stations Bregenzer Bucht and Zeller See are shallow (<60 m) with lower densities of coregonid fish. B. longimanus was present in the pelagic zone from mid May to November at all stations. L. kindtii generally appeared slightly later in the year than B. longimanus. Peak abundances of B. longimanus with more than 5000 ind. m^–2 (Langenargen-Arbon) appeared in late spring. During summer, when predation pressure by fish is high, abundance of B. longimanus decreased gradually. In contrast, L. kindtii displayed maximum abundances with more than 18 000 ind. m^–2 (Zeller See) in summer. During the summer months, B. longimanus lived in greater depths than L. kindtii. Differences of vertical distribution between both species were most pronounced at the deep stations (Langenargen-Arbon, Fischbach-Uttwil). Analysis of stomachs of the planktivorous fish Coregonus lavaretus showed that B. longimanus was preferred by C. lavaretus over all other prey; Ivlev's selectivity index was 0.97–0.99. Due to this high positive selection for B. longimanus by coregonids, it is surprising that the cladoceran could maintain its existence since the first recordings in 1877. Spatial refuge or indigestible resting eggs may be the underlying mechanism for this phenomenon.     

Biology of the threespine stickleback,Gasterosteus aculeatus,and the blackspotted stickleback,G. wheatlandi,during their marine pelagic phase in the Bay of Fundy,Canada

Synopsis Two species ofGasterosteus were collected from the surface waters of 48 stations in the Bay of Fundy from 1979–1982. Densities varied widely according to locality and time of year (range: 1 fish 0.2 m^–2 to 1 fish 26.3 m^–2 of sea surface) but peaked within 30 km of the New Brunswick/Maine coast in July and August. A few adults were caught as far as 100 km from the coast in winter. Guts ofG. wheatlandi were significantly fuller (63.3 ± 0.6%) than those ofG. aculeatus (44.6 ± 0.1%) but there were no differences between male and female guts within species. Quantitatively, gut contents were very similar between species, consisting primarily of (99.4%): calanoid copepods (Centropages typicus Kroyer andEurytemora cfamericana Williams), copepod nauplii, cladocerans (Podon leuckarti Sars andP. polyphemoides Leuckart) and planktonic eggs (in decreasing order of importance).Gasterosteus wheatlandi ate significantly more calanoids thanG. aculeatus but all other prey types were eaten in similar amounts. Within species, there were no differences in the numbers of each prey type eaten over time. The most abundant calanoid species caught alongside the fishes in the sampling nets did not occur in the sticklebacks' diets; probably they were too large to be ingested.Gasterosteus aculeatus bred in inshore, brackish water ponds in late April. By late July most juveniles had migrated out into the Bay but a few remained until early September.Gasterosteus wheatlandi bred later in the brackish water ponds (late May) and juveniles began migrating into the Bay in early August; a few remained in the ponds until early September. Adults were found in the Bay alongside juveniles suggesting that individuals of both species may survive for a second year. In both species, growth rates of juveniles were greater in brackish water than in the sea.     

Intertidal territoriality and time-budget of the surgeonfish, Acanthurus lineatus, in American Samoa

Synopsis The herbivorous surgeonfish Acanthurus lineatus aggressively maintained feeding territories in the surf zone of the outer reef flat in American Samoa. Intertidal territories were re-established each morning, as well as after displacement by low tides or rough surf. Day-to-day site fidelity of recognizable individuals was high: 99.9% return rate per day for adults (15–20 cm), 99.6% for juveniles (8–13 cm), and 97.2% for recruits (2.5–5 cm). Fish fed on turf algae primarily in the afternoon (80% of available time), and spent 10% of their time on active territorial defense and 2–13% of their time on forays from their territory. On average, a fish defended its territory 1900 times daily and took 17 000 bites (= 7400 bites m^–2 d^–1), but rough surf reduced feeding by 60% and defense by 75%. High territorial defense requirements significantly reduced feeding rates. Although the distribution and behavior of this species in Samoa was in large part similar to that reported for it elsewhere (Australia, Indian Ocean), there were notable differences: in Samoa A. lineatus densities within colonies were greater (0.4 fish m^–2), territory size was smaller (2.3 m²), and defense rate against intruders was greater (2.5 attacks min^–1). These differences in Samoa may be related to their smaller body size, greater abundance or increased food supply caused by hurricane damage to reefs which has enhanced the algal turfs that A. lineatus feeds upon.     

Diet and annual production of two boreal river fishes following clearcut logging

Synopsis Diet and annual production of two cyprinid fishes in the Piscataquis River, Maine, were investigated before and after extensive deforestation of the surrounding watershed. Observed patterns were evaluated relative to post-logging changes in the benthic macroinvertebrate community, and to differences in ecomorphology between the two fishes. Prey selection was generally density-dependent for both predator species throughout the study, but significant exceptions to density-dependence were observed for several aquatic insect lifemode groups (e.g., swimmers, climbers, burrowers). This apparent preference/avoidance for certain prey taxa by fish predators could be adequately explained by differential availability (vulnerability) among prey taxa. Compared to the year before logging, significant post-logging changes in diet composition and gut fullness were observed for both blacknose dace,Rhinichthys atratulus, and creek chub,Semotilus atromaculatus, concomitant with post-logging changes in the availability of benthic prey. Annual production of blacknose dace was significantly lower (52.7 vs. 37.7 kg ha^–1 y^–1) in the year following logging. In contrast, annual production of creek chub increased significantly after logging, from 8.6 to 17.4 kg ha^–1 y^–1. Because post-logging changes in the physical environment were probably not sufficient to directly affect fish populations, we hypothesize that blacknose dace were morphologically constrained to feed on a declining aquatic prey resource, with a resultant decline in production. In contrast, the generalist creek chub switched to a non-aquatic prey resource (terrestrial arthropods) and no decline in production was observed for that species.     

Swimming patterns and behaviour of upriver-migrating adult pink (Oncorhynchus gorbuscha) and sockeye (O. nerka) salmon as assessed by EMG telemetry in the Fraser River,British Columbia,Canada

Little is known about the behaviour patterns and swimming speed strategies of anadromous upriver migrating fish. We used electromyogram telemetry to estimate instantaneous swimming speeds for individual sockeye (Oncorhynchus nerka) and pink salmon (O. gorbuscha) during their spawning migrations through reaches which spanned a gradient in river hydraulic features in the Fraser River, British Columbia. Our main objectives were to describe patterns of individual-specific swim speeds and behaviours, identify swimming speed strategies and contrast these between sexes, species and reaches. Although mean swimming speeds did not differ between pink salmon (2.21 BL s^–1) and sockeye salmon (1.60 BL s^–1), sockeye salmon were over twice as variable (mean CV; 54.78) in swimming speeds as pink salmon (mean CV; 22.54). Using laboratory-derived criteria, we classified swimming speeds as sustained (<2.5 BL s^–1), prolonged (2.5–3.2 BL s^–1), or burst (>3.2 BL s^–1). We found no differences between sexes or species in the proportion of total time swimming in these categories – sustained (0.76), prolonged (0.18), burst (0.06); numbers are based on species and sexes combined. Reaches with relatively complex hydraulics and fast surface currents had migrants with relatively high levels of swimming speed variation (e.g., high swimming speed CV, reduced proportions of sustained speeds, elevated proportions of burst speeds, and high rates of bursts) and high frequency of river crossings. We speculate that complex current patterns generated by river constrictions created confusing migration cues, which impeded a salmon's ability to locate appropriate pathways.     

Ecology and life history of the clown goby inhabiting the upper Banana River,Cape Canaveral,Florida

Synopsis Monthly collections of clown goby,Microgobius gulosus, were made from March 1984 through February 1985 at two stations located at the head of the Banana River, Brevard County, Florida, as part of the long-term environmental monitoring program at the John F. Kennedy Space Center. A total of 18921 fishes comprising eight families and 12 genera was collected.M. gulosus represented 6.4% of the total catch. Populations ofM. gulosus exhibited aggregation behavior, which varied in intensity depending on densities of individuals m^–2 and habitat characteristics. Capture data were best described by the negative binomial distribution. Mean estimates of individuals m^–2 ranged from 0.0 to a high of 22.1 during periods of peak recruitment. The total length-weight regression for all individuals measured was log₁₀ weight = –4.65 + 2.72 log₁₀ length.M. gulosus obtained a size of 35–40 mm TL the first year and 50–60 mm TL the second year. Total lengths of all specimens ranged from 11 to 71 mm. Young of the year first appeared in May with smallest individuals collected in June and July. A protracted spawning period was observed. Stomach and gut analyses revealed that crustaceans and annelids combined represented 65% and 66% of the total diet forM. gulosus from stations 1 (open) and 2 (impounded), respectively. However, differences in proportions of the two groups were present between the two stations. Crustaceans represented 47.1% of the diet for gobies collected at station 1 and annelids 40.8% for specimens collected from station 2. Fecundity was low with the mean number of ova being 305 ± 77.5 for females between 35 and 49 mm TL. Estimated mortality rate was approximately 95% annually.     

Distribution of Hexagenia nymphs and visible oil in sediments of the Upper Great Lakes Connecting Channels

As part of the study of the Upper Great Lakes Connecting Channels sponsored by the U.S. Environmental Protection Agency, the U.S. Fish and Wildlife Service examined the occurrence of Hexagenia nymphs and visible oil in sediments at 250 stations throughout the St. Marys River and the St. Clair-Detroit River system from May 14 to June 11, 1985. The mean density of Hexagenia nymphs per square meter averaged 194 for the total study area, 224 in the St. Marys River, 117 in the St. Clair River, 279 in Lake St. Clair, and 94 in the Detroit River. The maximum density of nymphs ranged from 1,081 to 1,164 m^-2 in the three rivers and was 3,099 m^-2 in Lake St. Clair. A comparison of nymph density at 46 stations where oil was observed in sediments physically suitable for nymphs showed that densities were lower in oiled sediments (61 m^-2) than in sediments without oil (224 m^-2). Densities of nymphs were relatively high at only four stations where oil was observed in sediments. In general, oiled sediments and low densities of nymphs occurred together downstream from industrial and municipal discharges.Contribution number 736 of the National Fisheries Research Center-Great Lakes, U.S. Fish and Wildlife Service, 1451 Green Road, Ann Arbor, MI 48105.     

Agar yield and quality of Gracilaria chilensis (Gigartinales,Rhodophyta) in tank culture using fish effluents

Tank cultivation of Gracilaria using fish effluents has permitted a production of 48 kg m^–2 yr^–1 and can reduce the dissolved nitrogen loads in the seawater. We report the yield, gel strength, gelling and melting point of agar from Gracilaria cultivated in tanks with seawater previously utilized in intensive, land-based salmon cultures and compared to a control using directly pumped seawater, over a study period of 22 months. The results show that the highest agar yield (20 to 22%) was obtained when Gracilaria was cultivated with pure seawater as compared to the fish effluents. The gel strength, gelling and melting point were higher in the agar obtained from algae cultured with fish effluents. During the spring, the gel strength, gelling and melting point increased in tanks with fish effluents and decreased in tanks with a supply of pure seawater.     

Population Dynamics of the Pink Salmon Oncorhynchus gorbuschain the Basin of the Sea of Okhotsk in the 1990s

In the 1990s, an extensive body of data was gathered on the size of the Oncorhynchus gorbuschapink salmon populations of the Sea of Okhotsk at all the main developmental stages. A significant increase in numbers was found for juvenile pink salmon migrating into the offshore regions of the Sea of Okhotsk and the Pacific waters around the Kurils: from 250–450 million in 1990–1991 to 807–1570 million fish in 1993–1999. The overall number of migrating pink salmon in even years sharply increased in 1994 up to 215 million fish. After 1994, this estimate exceeded the number of migrating pink salmon in preceding odd years. Ocean survival of juvenile pink salmon gradually declined. This may be related to changes in the North Pacific pelagic ecosystems.     

Use of radio telemetry and electrofishing to assess spawning by transplanted Atlantic salmon

The river Ingdalselva, which drains to the Trondheims fjord, has no local salmon population due to an impassable waterfall 500 m upstream from the outlet. In the period 1994–97, a total of 31 mature Atlantic salmon (19 females and 12 males) from the rivers Orkla (1994–96) and Vigda (1997) were radio-tagged and released in the river Ingdalselva before spawning. The main goal of the project was to find out whether the fish would stay and spawn in the river, and if the observations during the spawning period could be used to indicate where spawning had taken place. Some fish left the river shortly after release, but 77% of the fish stayed in the river during the spawning period in October. Most of the females (74%) spawned in the river, including multi-sea winter salmon of approximately 10 kg. Some fish remained at the site of the release, while others migrated downstream to hiding places where they stayed until spawning. Long distance upstream migrations were not observed. The observations of the fish during the spawning period were used to identify spawning areas. Electrofishing for salmon fry in the spawning areas the following year in all cases produced fry, and in two of the years salmon fry were also found in areas where no spawning activity had been registered. Radio-tracking was an efficient method for determining whether transplanted salmon would remain in a `new'n river during the spawning season and for locating spawning areas, particularly when the fish were tracked daily.     

In situ and laboratory studies on the behaviour and survival of Pacific salmon (genus Oncorhynchus)

Juvenile Pacific salmon display a marked surface water orientation during downstream migration, estuarine and nearshore coastal rearing phases. Many estuaries in British Columbia are vertically stratified with a shallow, well-defined halocline which can restrict the dispersion of wastes discharged into less saline surface waters and impose constraints upon aquatic organisms. In situ experiments in an estuary receiving a surface discharge of treated pulp mill wastes, revealed conditions which were lethal to underyearling salmon at, and below the halocline (4.0–6.5 m depth). Behavioural bioassays determined that juvenile chinook salmon were biased towards the water surface and avoided waters at depth. Dissolved oxygen was the variable which affected this distribution most significantly. Surface waters receiving effluent from another pulp mill were lethal to juvenile salmon within 350 m, and a significant vertical avoidance response occurred within 350–950 m of the outfalls. The behavioural response was significantly correlated with in situ temperature, pH and colour (effluent).As a complement to field experiments we developed a 4500 l water column simulator (WCS) to examine salmon behaviour in the laboratory. We investigated the surface water orientation behaviour of juvenile salmon in relation to variations in salinity and dissolved oxygen. Under simulated vertically stratified estuarine conditions, the fish moved freely between overlying fresh water and salt water. Induction of hypoxic conditions in fresh water elicited a downward distribution shift towards the halocline and oxygenated, but more saline, waters. Avoidance reactions (50% level) occurred consistently up to 7–8 mg · l^–1 dissolved oxygen. Salmon continued to examine the hypoxic freshwater zone despite sub-optimal conditions.     

Cultivation and carrageenan yield and quality ofKappaphycus alvarezii in the waters of Vietnam

The carrageenan-producing red algaKappaphycus alvarezii (Doty) Doty was brought to Vietnam from Japan in 1993. Branch fragments of this species were cultivated in a pond, lagoon, inlet and offshore in Vietnam for the first time. The best daily growth rate (DGR) of plants grown in the lagoon area attained 9–11 % day^–1 in May to June (cold season). The water temperature and salinity in this area ranged from 27.2–32.4 °C and 31.4–33.7 °C, respectively. DGR of plants grown in the inlet ranged from 7 to 9% day^–1 in June. Grazing by fish has been observed to occur in this area. The DGR of plants grown in the pond ranged from 5–6% in January–July, but decreased to less than 4% day^–1 in August (hot season). K. alvarezii in Vietnam showed a carrageenan yield of 18.8–24.6% and gel strength of 1566–1712 g cm^–2. These values are similar ones obtained fromK. alvarezii cultivated in the Philippines and Indonesia.     

Why do some fish do it younger than others? learning from experiments

Atlantic salmon in Maine were once abundant but have become depleted, and are listed as endangered under the federal Endangered Species Act. Historically, salmon numbers in Maine may have been as high as 100 000 adults, but habitat loss, pollution and overfishing have contributed to the decline of the species. In 2000, only 110 adults returned to spawn in Maine rivers. Maine produces c. 15 000 metric tons/year of aquacultured Atlantic salmon from a total of nearly 600 coastal net pens. Escapees from these pens may interact with the wild salmon. The dynamics of salmon populations under such conditions are poorly understood. In order to illuminate the role aquaculture may play in such a system, we have developed a model for simulating population trajectories for both wild salmon and competing populations derived from aquaculture escapes. The model simulates a small population of wild salmon based in a stream/estuary system, into which an aquaculture facility is losing fish to escapes. Biological parameters in the model were estimated as much as possible from data in the USFWS report on Maine salmon. We used the model to investigate the consequences of a variety of ecological interactions between the wild and cultured fish including competitive, genetic and disease effects. Initial results indicate that many of these effects allow the aquaculture‐derived population to supplant the wild fish, but that wild populations may still persist under some conditions.     

Comparison of allopatric cutthroat trout stocks with those sympatric with coho salmon and sculpins in small streams

Synopsis Juvenile stocks of allopatric (upstream of barrier falls) cutthroat troutSalmo clarki and those sympatric (downstream of barrier falls) with coho salmonOncorhynchus kisutch and sculpinsCottus spp., were sampled during the late summer period of low flows in six small coastal streams in British Columbia. The objective was to obtain and compare information on pattern of habitat use and fish size distribution of these two trout types. In most instances, density (n m^–2; g m^–2) of cutthroat trout was considerably greater in pools and glides in the allopatric than in the sympatric stocks. The sympatric salmonids were dominated by juvenile coho salmon in pools and cutthroat trout in riffles. Sympatric cutthroat trout constituted from 7 to 45 % of the salmonids present in pools and from 50 to 90% in riffles. Glides were areas of intermediate densities for both salmonids, although coho salmon was the more abundant species in most instances. The density of sculpins was high in all three habitat types, and frequently it exceeded that of coho salmon and cutthroat trout combined. Sympatric cutthroat trout consisted primarily of underyearling fish, whereas allopatric cutthroat trout consisted primarily of two or more age classes with a large proportion of them living in pools. When tested in a laboratory stream both types of cutthroat trout had similar habitat preferences and agonistic behaviours, with the exception that allopatric trout made greater use of cover and defended pools more intensely than sympatric trout when the flow was increased. The results of this study provide insight of potential impact of coho salmon juvenile transplants into stream segments supporting allopatric cutthroat trout.     

Effects of aluminum,calcium and low pH on egg hatching and nymphal survival of Cloeon triangulifer McDunnough (Ephemeroptera: Baetidae)

Laboratory experiments were conducted to assess the effects of aluminum, calcium and low pH on egg hatching and nymphal survival of the mayfly Cloeon triangulifer. Percent successful hatch (living nymphs breaking free of the chorion) decreased and percent partial hatch (nymphs dying attached to the chorion) increased with increasing acidity (pH 7.5–3.0). Most hatches occurring below pH 5.0 were partial hatches. Decreased time of exposure to acidic waters increased percent successful hatch and decreased percent partial hatch. Time to first hatch was not affected by pH. Eggs were incubated in acidic waters (pH 4.0 and 5.5) with additions of calcium (10 and 100 mg l^–1) and aluminum (100 and 500 g l^–1). Aluminum decreased percent successful hatch and increased percent partial hatch and calcium increased both percent successful hatch and percent partial hatch. Time to first hatch was increased by both aluminum and calcium. The 96 h LC50 for small nymphs was pH 4.75. Addition of aluminum (100 and 500 µg l^–1) to acidic waters (pH 4.0 and 5.0) reduced nymphal mortality by 8–22%. Toxic effects of low pH on egg hatching and early nymphs may contribute to the absence of mayflies from acidified habitats.Contribution No. 1469 of the Maine Agricultural Experiment Station, University of Maine, Orono, Maine 04469 USA.Contribution No. 1469 of the Maine Agricultural Experiment Station, University of Maine, Orono, Maine 04469 USA.