Tonle Sap Great Lake, in the lower Mekong River basin, contributes 60% of Cambodia's inland fisheries catch. There are four types of fisheries, including the middle-scale fishery. The major species comprising the middle-scale fishery are Henicorhynchus spp. (16%), a small-bodied fish caught mainly with small-mesh nets, and Cyclocheilichthys enoplos (13%), and Channa micropeltes (7%) which are caught with a variety of gear. Here we apply the Schaefer surplus production model to the middle-scale fisheries using monitoring data collected between 1995 and 1999. Model simulations determined that the maximum sustainable yield (FMSY) for Henicorhynchus spp., was obtained with the use of 47,206 gillnets (mesh size <50 mm) and 4269 and 1605 fishing boats for C. enoplos and C. microplestes, respectively. Over 1995–1999, there was a strong trend of increase in fishing effort and decrease of catch-per-unit-effort in the waters of Pursat, Siem Riep and Kampong Chnnang provinces. These provinces have large population centres located close to the fishing grounds; fishing in these areas should be more strictly regulated.

The rehabilitation of extirpated lake trout (Salvelinus namaycush) in the Great Lakes and Lake Champlain has been hindered by various biological and physiological impediments. Efforts to restore a lake trout fishery to Lake Champlain include hatchery stocking and sea lamprey control. Despite these management actions, there is little evidence of recruitment of naturally-produced fish in annual fall assessments. Spawning occurs at multiple sites lake-wide in Lake Champlain, with extremely high egg and fry densities, yet sampling for juvenile lake trout has only yielded fin-clipped fish. To investigate this recruitment bottleneck, we assessed predation pressure by epi-benthic fish on emergent fry on two spawning reefs and the subsequent survival and dispersal of fry in potential nursery areas. Epi-benthic predators were sampled with 2-h gillnet sets at two small, shallow sites in Lake Champlain throughout the 24-h cycle, with an emphasis on dusk and dawn hours. In total, we documented seven different species that had consumed fry, with consumption rates from 1 to 17 fry per stomach. Rock bass and yellow perch dominated the near-shore fish community and were the most common fry predators. Predator presence and consumption of fry was highest between 19:00 and 07:00. Predators only consumed fry when fry relative abundance was above a threshold of 1 fry trap^-1 day^-1. We used an otter trawl to sample for post-emergent fry adjacent to the reef, but did not capture any age-0 lake trout. Due to the observed predation pressure by multiple littoral, species on shallow spawning reefs, lake trout restoration may be more successful at deep, offshore sites.

Little is known of mudpuppy (Necturus maculosus) population structure and ecology; some populations in the Great Lakes are thought to be in decline. Mudpuppies are the obligate hosts for the mudpuppy mussel (Simpsonaias ambigua), a species that is endangered in Canada and in many Great Lakes states. We surveyed mudpuppies from the Sydenham River, the only known Canadian locality of the mudpuppy mussel, in order to generate information on relative density, deformity rates and population age/size structure and used this information to compare them to known mudpuppy populations from Great Lakes sites in the Detroit River, Lake St. Clair and Long Point in Lake Erie. Deformity rates were elevated at some sites in the Sydenham River. The relative density of mudpuppies in the Sydenham River was lower than that of other Great Lakes sites and their age was skewed towards younger individuals. Although at lower densities than at other Great Lakes sites, the mudpuppy population in the Sydenham River appears stable and is showing signs of recruitment which bodes well for the future of the mudpuppy mussel population of the river.

To better understand the feeding ecology of two important Laurentian Great Lakes prey species, rainbow smelt Osmerus mordax and emerald shiners Notropis atherinoides, we quantified the diet composition, selectivity, daily ration, and diet overlap of both species in offshore central Lake Erie during May through October 2005, which spanned a period of severe hypolimnetic hypoxia (<2 mg O₂/L). Rainbow smelt fed upon a variety of prey taxa, including zooplankton, chironomid pupae and larvae, and fish, whereas emerald shiners primarily consumed cladocerans, if available. In turn, diet overlap between rainbow smelt and emerald shiners was low except during September when hypolimnetic hypoxia reduced rainbow smelt access to benthic prey. Rainbow smelt most frequently selected chironomid pupae, while emerald shiners generally selected pupae or large predatory cladocerans (Leptodora or Bythotrephes). Daily ration and individual consumption by rainbow smelt were 54–68% less during hypoxia than at the same site during stratified pre-hypoxic or mixed post-hypoxic conditions. Although emerald shiner daily ration and individual consumption decreased between pre-hypoxic and hypoxic periods, it continued to decrease during the post-hypoxic period, suggesting that reduced consumption may not have been linked to hypoxic conditions. Ultimately, our findings suggest that emerald shiners are as important regulator of zooplankton abundance in the Great Lakes as rainbow smelt, given their potentially high mass-specific consumption rates, selectivity and diet patterns, and current high abundance.

The establishment of dreissenid mussels in the Great Lakes has been implicated in causing a resurgence of benthic macroalgae, notably Cladophora glomerata (L.) Kützing. The seasonal proliferation and nutrient retention of epiphyton on Cladophora may be important to littoral zone processes. In this study, peak diatom epiphyte density (601 diatoms µg^-1 dry mass) co-occurred with peak Cladophora biomass. The exceptionally adhesive Cocconeis pediculus came to dominate the epiphyton to the near exclusion of all other diatoms by early summer, indicating powerful constraints on the epiphyte assemblage. There was a strong relationship between epiphytic silica content and epiphyte abundance, but the relationship was different between seasons. In the spring and fall when Cladophora was growing vigorously, silica content of the diatom epiphytes was 20.1–25.6 pmol Si diatom^-1. In the summer, during Cladophora senescence, silica content was 6.5–10.0 pmol Si diatom^-1 and valves were visibly thinner. These observations suggest that diatom epiphytes may be limited by Si during peak Cladophora biomass. Areal estimates of silica content of Cladophora epiphyton was strongly related with Cladophora biomass: at 2 m depth, silica content increased from 22.7 mmol Si m^-2 in the spring to 490 mmol Si m^-2 during peak Cladophora biomass. Silica content can be a valuable proxy for diatom epiphyte abundance if the vitality of the Cladophora substrate is considered. Future work needs to assess the seasonality of pelagic versus benthic silica demand and the interannual variability of epiphyte silica content to assess how changes in Cladophora biomass may affect nearshore Si cycling.

Coastal wetland seed banks exposed by low lake levels or through management actions fuel the reestablishment of emergent plant assemblages (i.e., wetland habitat) critical to Great Lakes aquatic biota. This project explored the effectiveness of using portable, water-filled cofferdams as a management tool to promote the natural growth of emergent vegetation from the seed bank in a Lake Erie coastal wetland. A series of dams stretching approximately 450 m was installed temporarily to isolate hydrologically a 10-ha corner of the Crane Creek wetland complex from Lake Erie. The test area was dewatered in 2004 to mimic a low-water year, and vegetation sampling characterized the wetland seed bank response at low, middle, and high elevations in areas open to and protected from bird and mammal herbivory. The nearly two-month drawdown stimulated a rapid seed-bank-driven response by 45 plant taxa. Herbivory had little effect on plant species richness, regardless of the location along an elevation gradient. Inundation contributed to the replacement of immature emergent plant species with submersed aquatic species after the dams failed and were removed prematurely. This study revealed a number of important issues that must be considered for effective long-term implementation of portable cofferdam technology to stimulate wetland seed banks, including duration of dewatering, product size, source of clean water, replacement of damaged dams, and regular maintenance. This technology is a potentially important tool in the arsenal used by resource managers seeking to rehabilitate the functions and values of Great Lakes coastal wetland habitats.

The multi-scale nature of streams, rivers, and inland lakes is well documented, although relationships between the ecology of Great Lakes nearshore areas and shoreline processes are generally poorly described. Given the high levels of development pressure currently exerted on Great Lakes shorelines, we sought to determine whether patterns exist between measures of shoreline development quantified at multiple spatial scales and adjacent fish community measures. We expected that fish measures for nearshore areas immediately adjacent to intact versus modified shorelines would differ as a result of the greater buffering capacity of the intact shorelines. Further, we expected anthropogenic shoreline factors to act cumulatively in combination with prevailing currents to influence fish communities in downdrift nearshore areas. Our results indicated that a few shallow water and nearshore fish community measures exhibited significant patterns that may be attributable to immediately adjacent shoreline characteristics. In addition, several fish measures were related to urban-residential land uses and shore structure numbers of updrift shoreline areas, suggesting that cumulative anthropogenic factors operating over larger spatial scales also influence local fish communities. Based on these results, we argue that there is critical need for multi-scale management strategies for shorelines that address the potential for both local and cumulative, larger-scale environmental impacts relative to local nearshore biota.

The trophic roles of key Ponto-Caspian invaders (quagga mussels Dreissena bugensis, amphipods Echinogammarus ischnus and round goby Apollonia melanostomus) within the littoral food web of eastern Lake Erie were quantified using stable isotopes (δ¹³C, δ¹⁵N). A dual stable isotope parameter search with a mass balance component was used to assess the isotopic importance of quagga mussels and amphipods as dietary items to two size classes of round goby. The utility of the mass balance simulation was also evaluated as a tool to approximate isotopic contributions of feasible prey and identify gaps incurred by “missing” prey items not included in the sampling. The mass balance dietary simulation, confirmed by stomach content data, indicated that isotopically important prey to small round goby (<11.2 cm) were chironomids and Ponto-Caspian amphipods, while large round goby (≥11.2 cm) showed strong preference for quagga mussels. The dietary mass balance simulation output also supported the isotopic importance of round goby to the somatic growth of smallmouth bass, rock bass and freshwater drum. The isotopic mass balance output for yellow perch was more ambiguous, which may be in line with their known broadly omnivorous diet. The white bass output was in line with published data indicating increasing consumption of round goby for this species, while the brown trout output strongly favoured alewife isotopic contributions. However for white perch and walleye, the mass balance simulations were not in line with their known published diets in Lake Erie, probably due to a lack of key prey items in the sample set (e.g. zooplankton for white perch and shiner species for walleye). As expected, the Ponto-Caspian species have integrated themselves into the littoral food webs, and the “quagga mussel-round goby-smallmouth bass” food chain forms one of the key components within the trophodynamics of Lake Erie.

We determined the distributions of Chinook salmon and rainbow trout by describing seasonal mean vertical and bathymetric catch depths from 1997 to 2005 using angler creel surveys. We developed and applied a cross-validated model of Lake Ontario temperatures to determine the water temperatures associated with these distributions. During April, Chinook salmon and rainbow trout were found nearshore at a bathymetric depth of 20 m. However, rainbow trout were caught at shallower vertical depths (4 to 6 m) than Chinook salmon (8 to 10 m). Both species moved deeper and farther offshore during May, June, and July. Vertical catch depths were similar, but rainbow trout were found further offshore (40 to 65 m bathymetric depth) than Chinook salmon (35 to 50 m bathymetric depth) during June, July and August. During September, Chinook salmon moved closer to shore (25 to 35 m bathymetric depth) and to shallower depths (9 to 12 m), consistent with river mouth staging associated with spawning. Rainbow trout remained offshore (45 to 60 m bathymetric depth) in deeper water (11 to 16 m). The species occupied significantly different spatial habitats during April, August, and September. Mean catch temperatures of both species were similar and increased seasonally to 13 to 14 °C during August and September. Rainbow trout were caught at cooler temperatures than Chinook salmon during June and July. The estimated temperature distributions agree with independent field studies but are different then previously assumed in bioenergetic models.

In the 1990s, the Lake Ontario ecosystem was dramatically altered due to continued invasions of exotic species including dreissenid mussels and predatory cladocerans. We describe the diet and biomass of prey in the stomachs of adult (≥109 mm TL) and sub-adult (<109 mm TL) alewife (Alosa pseudoharengus) in 2004 and 2005 across seasons and depths and compare our results to data from 1972 to 1988. During 2004 and 2005, adult alewife consumed primarily zooplankton prey at bottom depth zones <70 m and primarily Mysis at bottom depth zones >70 m. Mysis dominated the diets of adult alewife in all seasons except during the summer of 2004 when zooplankton dominated. Mysis dominated the diets of sub-adult alewife during early and late spring and zooplankton dominated the diets in summer and fall. Bythotrephes and Cercopagis were observed in the diets of both sub-adult and adult alewife. Diporeia was observed only rarely in adult alewife diets. The biomass of prey in alewife stomachs varied seasonally and increased with bottom depth for adult alewife. Alewife diets in 2004–2005 differed from those in 1972 and 1988 with an increase in the prevalence of Mysis, and a decline in the prevalence of zooplankton. The biomass of prey in adult alewife stomachs declined in 2004 and 2005 compared to 1972 and 1988, at bottom depth zones <70 m but not at bottom depth zones >70 m suggesting reduced food availability closer to shore. We hypothesize that consumption levels at the shallower depth zones, as indicated by very low biomass of prey in alewife stomachs, may not be sufficient to sustain alewife growth. The increased prevalence of Mysis and common occurrence of predatory cladocerans in the diet of alewife means that alewife have shifted to a higher trophic position.

The Lake Champlain basin contains substantially fewer exotic species (N=48) than the Great Lakes (N>180), in part due to its isolation from commercial traffic. Exotic species have been introduced by authorized and unauthorized stocking, bait buckets, use of ornamental plants, and through the Champlain and Chambly canals that link the lake to the Hudson River, Mohawk River, Erie Canal, and the Great Lakes. Several species, such as water chestnut and zebra mussels, have had severe ecological, economic, and nuisance effects. The rate of appearance of new species increased in the 1990s, potentially as a result of increasing activity in the basin, improved water quality in the Champlain Canal, and increased sampling. Efforts to slow the introduction of new species have focused on public education and legislation to reduce bait bucket introductions and quarantine undesirable plants; however, the major remaining vector for introductions is the Champlain Canal. An estimated 20 species have entered the lake via canals, of which at least 12 used the Champlain Canal, and numerous species in the connected drainage systems could still enter via this route; some are already in the Erie Canal. Most recently (2008), the Asian clam was discovered two locks below Lake Champlain. The Lake Champlain canals also function as a conduit for exotic species exchange between the Hudson River, St. Lawrence River, and Great Lakes. The potential for future introductions could be reduced by a biological barrier on the Champlain Canal, and additional emphasis on public education.

We assessed round goby (Apollonia melanostoma) density and size structure in two sections of the Bay of Quinte (Lake Ontario) that had been invaded by this species two years apart. Round goby density was assessed with 50 m linear transects, recorded with an underwater video recording apparatus developed for this study that included a depth sounder for maintaining a fixed distance above the substrate. The highest mean round goby densities were observed in the shallowest depth zone (1.5–3 m) at both sites, but there were differences between the sites in the habitat types where the highest densities occurred and there were no significant density differences among habitat types at either site (rock with sparse vegetation, mud with sparse vegetation, sand/mud with moderate vegetation cover). In the upper bay, mean body length of round gobies declined with depth, whereas in the lower bay, mean round goby length was greatest in the deepest zone. Mean body length of round gobies did not differ significantly by habitat type in either section of the bay.

Surveillance data collected over the past 150 years are compiled and analyzed to identify chloride trends in the Laurentian Great Lakes. These data indicate that chloride levels started rising in the mid-19th century and began accelerating in the early twentieth century. Lake Superior's and Lake Michigan's concentrations have continued to increase steadily and currently stand at their maximum recorded levels. In contrast, lakes Huron, Erie and Ontario reached peak levels between 1965 and 1975, but then began to decline. However, recent data indicate that the chloride concentrations in these lakes are now increasing again. Because loading data are not readily available, a mass-balance model is employed to estimate the chloride inputs required to account for the concentration trends. This inverse analysis yields computed load reductions that are consistent with reported industrial load reductions during the last three decades of the 20th century. Hence, it appears that the improvements were for the most part attributable to industrial controls. The model is also used to predict that if loads are held fixed at 2006 levels, concentrations in all lakes will continue to increase with the most dramatic rise occurring in Lake Michigan which will ultimately approach the level of Lake Erie.

Since 2004, population density of the large hypolimnetic calanoid Limnocalanus macrurus Sars. has increased dramatically in Lake Michigan. The average summer biomass of this species between 2004 and 2006 was roughly three times that of the period 1984–2003, and at levels unprecedented in our 22-year dataset, making L. macrurus the dominant zooplankter in the lake in terms of biomass. These increases have been accentuated by coincident population declines of the main daphnid, Daphnia mendotae, in the lake with the result that in 2006, L. macrurus accounted for 75% and 50% of the large (>0.9 mm) crustacean biomass in the northern and southern basins of Lake Michigan, respectively. The increases in L. macrurus populations have closely coincided with equally dramatic increases in summer water clarity. Recent extinction coefficients are among the lowest recorded for the lake, and deepening light penetration has permitted increases in the size of the deep chlorophyll layer. In addition, planktivorous fish populations have declined coincidently with the increases in L. macrurus. It seems likely that an increase in sub-epilimnetic production has resulted in increased food resources for the deep-living L. macrurus, while low planktivore abundances have reduced predation loss, permitting L. macrurus to respond to these increases in sub-epilimnetic production.

Systematic water quality research and monitoring has been on-going in Hamilton Harbour since 1987 in response to the Remedial Action Plan (RAP) for this Area of Concern (AOC). Here we present a spatiotemporal analysis of water quality in the harbour and its biological response from 1987 to 2007. Overall nutrient concentrations have decreased by 16 (SRP), 26 (NH_3-Tot) and 36% (TP) in the harbour, chl a concentrations have decreased by 16% and NO_3/2 concentrations have increased by 27%. Hypoxia in the hypolimnion of Hamilton Harbour remains a common occurrence despite improvements in surface water quality conditions. Seasonal patterns in water quality in Hamilton Harbour are mainly driven by biological activity and show typical patterns observed in dimictic nutrient rich lakes. There is systematic spatial variability in water quality in the harbour which is related to the proximity of point and non-point sources; however, there is coherence among all stations sampled and similar temporal trends were observed for all stations. The biological response in the harbour suggests that phosphorus limited algal growth is becoming more prevalent in Hamilton Harbour and the rate of improvements in water quality should accelerate in the near future following further reductions in phosphorus loadings.

Alternative male reproductive tactics are taxonomically widespread. In such species, parental, or conventional, males express secondary sexual characteristics, court females and guard offspring, while smaller parasitic or sneaker males avoid the costs of courtship and parental care by performing sneak fertilizations. Theory predicts that sneakers will invest more in testes mass and produce more competitive ejaculates than parentals because sneakers always experience sperm competition while parental males experience sperm competition only when a sneaker is present. Here we present convergent lines of evidence supporting the existence of alternative male reproductive tactics in round gobies (Apollonia melanostoma, formerly Neogobius melanostomus), a recent invader in the Great Lakes. Dark morph males exhibited secondary sexual characteristics, were larger and had higher plasma 11-ketotestosterone concentrations than light morphs, while light morph males invested more in ejaculates (both testes mass and sperm density). Both male morphs had enlarged urogenital papillae, but papillae were relatively longer in light morph males. Sperm tail length did not differ between morphs, and sperm from dark morphs swam faster than sperm from light morphs. Our data strongly argue for the presence of alternative tactics in round gobies, support some predictions from sperm competition theory and align with empirical observations in other taxa. For species of concern like the invasive round goby, it is critical to consider such evidence of alternative male mating tactics when constructing population growth models and assessment of invasion success and impacts.

We examined thiaminase activity in dreissenid mussels collected at different depths and seasons, and from various locations in Lakes Michigan, Ontario, and Huron. Here we present evidence that two dreissenid mussel species (Dreissena bugensis and D. polymorpha) contain thiaminase activity that is 5–100 fold greater than observed in Great Lakes fishes. Thiaminase activity in zebra mussels ranged from 10,600 to 47,900 pmol g^-1·min^-1 and activities in quagga mussels ranged from 19,500 to 223,800 pmol g^-1 ·min^-1. Activity in the mussels was greatest in spring, less in summer, and least in fall. Additionally, we observed greater thiaminase activity in dreissenid mussels collected at shallow depths compared to mussels collected at deeper depths. Dreissenids constitute a significant and previously unknown pool of thiaminase in the Great Lakes food web compared to other known sources of this thiamine (vitamin B₁)-degrading enzyme. Thiaminase in forage fish of the Great Lakes has been causally linked to thiamine deficiency in salmonines. We currently do not know whether linkages exist between thiaminase activities observed in dreissenids and the thiaminase activities in higher trophic levels of the Great Lakes food web. However, the extreme thiaminase activities observed in dreissenids from the Great Lakes may represent a serious unanticipated negative effect of these exotic species on Great Lakes ecosystems.

Daphnia lumholtzi Sars, an exotic tropical/subtropical cladoceran from Australia, southeast Asia, and Africa, was newly found in Lake St. Clair in a vertical tow sample taken at 3 m depth on 25 July 2007. The species was previously found in 1990/1991 in some reservoirs in the southern United States from where it colonized many waters north to the Great Lakes. In 1999, it was found in Lake Erie. This cladoceran had a density of 117 individuals/m³ when we collected it from Lake St. Clair and it was represented by both females (95.12%) and males (4.88 %). It seems that D. lumholtzi will continue to expand its distribution area in the Great Lakes.

A system for visual surveys of bottom fishes, assembled from readily available components, is described. A camera and a depth sounder transducer are mounted on a towed body. The camera captures the images, and the depth sounder measures the distance between the camera and the bottom. A video recorder is used to record the data—the video channel for the image, and the audio channel for distance information, which is dictated from the depth sounder display into a microphone. A relationship between distance and magnification is used to estimate the width of the survey path and the size of surveyed objects. A comparison of round goby (Apollonia melanostoma) density estimated by repeated seining and use of the underwater visual apparatus showed that the smallest individuals are underrepresented by the visual assessment, but the visual method detects about 85% of the 50 mm TL or larger gobies.