Bottom set gill nets were used to describe and track the fish community in northeastern Lake Ontario from 1992–2012. Six fixed, depth-stratified transects, spread more or less evenly from the mouth of the St. Lawrence River in the Kingston Basin to Brighton in central Lake Ontario, were sampled annually during summer. The balanced sampling design provided a broad picture of the warm, cool and coldwater fish community inhabiting open-coastal waters out to about 30 m water depth. Catch results were summarized by geographic area and depth strata to describe species distribution patterns, and presented graphically to illustrate annual abundance trends of the most important fish species (Alewife, Lake Trout, Yellow Perch, Walleye, Round Goby, Lake Whitefish, Brown Trout, Rock Bass, Smallmouth Bass, Chinook Salmon, Burbot, Cisco and Round Whitefish). Many of these dominant species showed peak abundance levels in the early 1990 s followed by decline. Of particular note, members of the coldwater benthic-oriented species assemblage, having all declined dramatically in the 1990s, remain at very low abundance levels, and their future prognosis appears bleak.
Fish community sampling in the open-coastal waters of northeastern Lake Ontario, employing bottom set gill nets with a graded series of mesh sizes, began in the 1970s. This sampling complemented previously established fish community sampling in the Bay of Quinte (Hurley, 1986a; Hoyle et al., 2012) and in the deep waters of eastern Lake Ontario's Kingston Basin (Christie et al., 1987). Initially, a variety of relatively shallow depths at several locations were sampled, and information collected was used largely to help manage the local commercial fishery that was focused primarily on Yellow Perch (Perca flavescens) at that time. The sampling design was expanded and standardized in 1986. The standardized design included summer-time depth-stratified gill netting that ranged from shallow water near shore out to about 30 m water depth, and thereby sampled a broad range of warm, cool and cold water fish species. Three areas, located in Lake Ontario's Kingston Basin (Melville Shoal, Grape Island and Flatt Point) were sampled in 1986 and 1987. Three more areas were added, to the west, in 1988 (Rocky Point, Wellington and Brighton; Figure 1). The gill net gear was modified (net specifications and material) in 1992; thereafter, no significant changes to the program were made.
The abundance of several key fish species increased in northeastern Lake Ontario during the 1980s. These species included native Walleye (Sander vitreus) (Bowlby et al., 1991) and Lake Whitefish (Coregonus clupeaformis) (Casselman et al., 1996; Hoyle et al., 1999). Both species existed only at remnant levels during the 1970s but their populations resurged during the 1980s. Lake Trout (Salvelinus namaycush) was extirpated from Lake Ontario by the 1950s (Christie, 1973; Christie et al., 1987). Lake trout numbers were built-up during the 1980 s thanks to massive rehabilitation stocking efforts, and the simultaneous control of the parasitic Sea Lamprey (Petromyzon marinus) (Christie et al., 1987; Owens et al., 2003).
The individual species that resurged in the 1980s showed dramatic declines in abundance in the 1990s (Hoyle et al., 2008). A common theme related to changes in fish abundance in the 1990 s appears to be changes in foodweb dynamics, possibly dreissenid-induced but also coincident with an overall decline in primary productivity (Mills et al., 2003). The standardized gill net sampling program, focusing on a broad range of water depths and temperatures in the open-coastal waters of northeastern Lake Ontario, should provide excellent abundance and biological data for these and other warm, cool and coldwater fish species, and hence, to facilitate detection of long-term ecosystem change. Here, I examine the fish community broadly to examine for common abundance trend patterns among a variety of species since 1992. Specifically, the objectives of this article are to: (1) document and describe fish species composition and species-specific distribution and abundance patterns and (2) examine temporal abundance trends among important species, in the open-coastal waters of northeastern Lake Ontario out to 30 m water depth, 1992–2012.
A consistent and balanced gill net sampling design was employed annually during summer from 1992–2012 at six depth-stratified sampling transects (hereafter referred to as sampling areas) spread more or less evenly from the mouth of the St. Lawrence River in the Kingston Basin to Brighton in central Lake Ontario (from east to west: Melville Shoal, Grape Island, Flatt Point, Rocky Point, Wellington and Brighton; Figure 1). Results are used here to provide a broad picture of warm, cool and coldwater fish assemblages inhabiting open-coastal waters out to 30 m water depth. Although gill net catch data were available for years prior to 1992, and longer time-series have been reported previously for individual fish species, e.g. Lake Whitefish and Walleye (Hoyle et al., 2008) and fish communities (Casselman and Scott, 2003), only post-1991 data were considered here. In addition to changes in sampling locations prior to 1992, gill net gear configuration and material were changed and standardized in 1992; therefore, the approach used in the present work avoided the use of species-specific catch conversion factors at a time when many species were undergoing major changes in population status including their abundance.
Five water depth strata (5–10 m, 10–15 m, 15–20 m, 20–25 m, and 25–30 m water depth) were sampled within each of the six geographic sampling areas. Preliminary investigation at sampling depths less than 5 m showed that fouling of the gill nets with filamentous algae was excessive; further attempts to sample at depths less than 5 m were abandoned. Each sampling area and depth was visited two or three times during summer. During each visit, two or three bottom set standard gill net gangs were deployed within each geographic area and depth strata and left overnight; set durations ranged from 18–24 h. Gill net gangs were set parallel to bottom contours within depth strata, and an average bottom depth over the length of the gill net recorded. Water temperature was measured at this average bottom depth.
Each gill net gang consisted of a graded series of ten monofilament gill net panels of mesh sizes ranging from 38 mm (1½ in) to 152 mm (6 in) stretched mesh with 13 mm (½ in) intervals, arranged in sequence. Each of the mesh sizes (panels) was 15.2 m (50 ft) in length except for the 38 mm mesh size which was 4.6 m (15 ft). Therefore the total gang length was 141.7 m (465 ft). The use of the shorter 38 mm gill net panel was related to minimizing the processing time required to deal with large numbers of small fish species, especially Alewife (Alosa pseudoharengus) and Yellow Perch, caught in this smallest mesh size. Gill net gangs were 2.4 m in height and were connected in series (i.e. cork lines and lead lines attached) but were separated by a 15.2 m (50 ft) spacer to minimize “leading” of fish. Fish from each gill net catch were identified to species, counted, and weighed in aggregate. Catches were summed across the ten mesh sizes from 38–152 mm. The catch in the 38 mm mesh size was multiplied by 15.2/4.6 prior to summing the ten mesh sizes. Therefore, all reported catches are intended to represent the total catch in a 152 m (500 ft) gill net gang. Mean catch was first determined for the two or three gangs set at each area-depth combination sampled during each visit. This mean was considered to be a replicate for further data summaries.
An overall (1992–2012) species-specific mean catch, by number and weight, as well as frequency of occurrence were determined, then an index of relative importance (IRI) following Hoyle et al. (2012) was used to quantify the relative dominance status of each fish species in the northeastern Lake Ontario fish community ((IRI = % by number +% by weight) × (% frequency of occurrence)). Also, overall species-specific mean fish lengths (fork length for species with a forked caudal fin, otherwise total length) with 10th and 90th percentiles were calculated to describe the size range of fish caught. Mean species-specific abundance (by number) results were summarized for selected most important species by geographic area and depth. Relative standard errors (RSE) were presented to generally assess variability and relative precision. Mean water temperature and depth of capture, weighted by the catch per gill net (by number), were calculated and used to assess species-specific temperature and depth preferences. The 10th and 90th percentiles of mean water temperatures were calculated to assess the breadth of water temperatures at the depth of capture among selected most important fish species. Note that, although Round Goby (Neogobius melanostomus) were not captured prior to 2003, all years (1992–2012) were included for calculation of indices. Annual abundance (catch per gill net; general linear model with geographic area and depth strata as factors and water temperature, at the depth of capture, as a covariate; StatSoft, Inc., 2007) was presented graphically to illustrate temporal trends of selected most important fish species. The annual mean weight of an individual fish was also calculated for Alewife to aid in the interpretation of change in this species' abundance.
Species composition and relative importance
A total of 1315 gill net samples were made during the 1992–2012 time-period. Thirty-three different fish species were captured (Table 1). Alewife and Yellow Perch dominated the catches numerically but in terms of the index of relative importance (IRI), Alewife, Lake Trout, Yellow Perch, Walleye, Round Goby, Lake Whitefish, Brown Trout (Salmo trutta), Rock Bass (Ambloplites rupestris), Smallmouth Bass (Micropterus dolomieu), and Chinook Salmon (Oncorhynchus tshawytscha) were the ten most important species. The most important small-bodied species were Alewife, Yellow Perch and Round Goby, and the most important large-bodied species were Lake Trout, Walleye and Lake Whitefish (Table 1).
Geographic and depth distribution
The geographic and depth distributions of catches are given in Tables 2 and 3 for selected most important species. Alewife was ubiquitous in terms of both geographic and depth distributions. So too was Yellow Perch generally, although very few were caught at Rocky Point and catches tended to decline with increasing depth. Lake Trout was widespread geographically and abundance increased with sampling depth; highest catches occurred at the deepest depth strata (25–30 m). Walleye, Rock Bass and Smallmouth Bass were most abundant at Melville Shoal and Grape Island, and their abundances decreased with increasing sampling depth. Walleye and Smallmouth Bass were also common at Rocky Point. Round Goby abundance was variable among sites with no geographic distribution pattern. Round Goby were consistently caught at all depths although abundance was lowest at the shallowest sampling depth (5–10 m). Lake Whitefish was most abundant at Flatt Point and Grape Island, and generally more abundant at the four eastern-most geographic areas, and much less abundant at Wellington and Brighton in the west. Lake Whitefish was most abundant at the two deepest sampling depth strata (20–25 and 25–30 m). Brown Trout was caught occasionally in all geographic areas; lowest catches were at Melville Shoal. Brown Trout was most abundant at the intermediate sampling depth strata (15–20 m); catches were lower in shallower and deeper depth strata. Chinook Salmon abundance generally increased from east to west with highest catches occurring at Brighton. Chinook Salmon was most abundant at the 15–20 and 20–25 m depths. Cisco (Coregonus artedi) was most abundant at Flatt Point; Cisco abundance was highest at the 20–25 m sampling depth. Burbot (Lota lota) was most abundant at Rocky Point and Wellington; Burbot abundance was highest at the two deepest sampling depths (20–25 and 25–30 m). Round Whitefish (Prosopium cylindraceum) was only captured at the two western-most areas, Wellington and Brighton. Round Whitefish was most abundant at the 20–25 m depth strata.
Preferred temperature and depth
Mean water temperature decreased with increased depth sampled from 18.0 °C at the shallowest depth strata (5–10 m) to 10.5 °C at the deepest depth (25–30 m) (Table 3). For selected important fish species, mean water temperature and depth of capture ranged from 10.5 °C and 24.5 m respectively for Lake Whitefish to 18.6 °C and 10.6 m for Smallmouth Bass (Table 4, Figure 2). Three multi-species groupings were apparent. In addition, to Lake Whitefish, species that tended to be caught in colder, deeper water included Round Whitefish, Lake Trout, Cisco and Burbot. In addition to Smallmouth Bass, species that tended to prefer warmer, shallower water included Walleye and Rock Bass. Brown Trout were found at intermediate water temperatures and depths. Brown Trout was captured over the most narrow temperature range. Chinook Salmon tended to be captured at temperatures and depths in-between Brown Trout and the cold, deep water group of species. Yellow Perch, Alewife and Round Goby were caught over the broadest range of water temperatures and depths.
Alewife abundance was stable from 1992 to 2004 then increased steadily through 2011, particularly after 2008. Alewife abundance declined in 2012 but overall, abundance remained high (Figure 3). The mean weight (g) of an individual alewife followed the same pattern as Alewife abundance. Lake Trout abundance was very high in the early 1990 s, declined precipitously from 1995 to 2005 then remained steady at a low level. Yellow Perch abundance declined after 1993, remained steady at moderate abundance levels through 2003, increased somewhat for several years, then declined to lowest observed abundance levels for the last two years. Walleye abundance declined over the 1992 to 2000 time-period, was steady until 2005, and then increased slightly over the remaining years. The invasive Round Goby first appeared in gill net catches in 2003. Round Goby abundance increased quickly, peaked in 2007 then declined. Lake Whitefish abundance peaked in 1993, declined through 2005, and then remained steady at a low level. Brown Trout abundance was variable with no clear trends although some of the highest catches occurred in the most recent few years. Smallmouth Bass and Rock Bass (Rock Bass abundance is not shown in Figure 3 but highly correlated with Smallmouth Bass; r = 0.61, p = 0.003, N = 21 years), abundance levels generally followed similar trends. Abundance declined from high levels in 1992 to lowest levels in 1996, increased through the late 1990s, declined again over one or two years, and then remained steady at moderate levels of abundance. Chinook Salmon abundance was highest in the early 1990s, lowest in the late 1990s and variable but with no clear trend thereafter. Burbot abundance levels showed moderate peaks in 1996 and 2002, declined after 2002, and then remained steady at a low level. Cisco abundance was at a peak in the early 1990s, declined precipitously through the mid-1990s, then remained at a low level of abundance. Round Whitefish abundance was relatively high from 1992 to 1996 and then declined and remained steady at a low level (Figure 3).
The gill net sampling program was conducted annually during the summer months. During this time of the year, the water column is thermally stratified in northeastern Lake Ontario. Typically the thermocline is established near the 15–20 m water depth range. The sampling depth stratification used in this study encompassed warm, shallow habitat near shore and cold water habitat out to 30 m water depth. This ensured that a wide variety of warm, cool and coldwater fish species were caught. Species-specific thermal habitat preferences were reflected in the water depths and temperatures that species were caught in this study. Thermal habitat partitioning was apparent. Smallmouth Bass, Rock Bass and Walleye tended to be captured in shallow, warm water (5–10 m; 15–22 °C). Lake Whitefish, Round Whitefish, Lake Trout, Cisco and Burbot tended to be caught in deeper, colder water (20–30 m; 6–16 °C). Brown Trout tended to be captured at a relatively narrow range of intermediate water depths and temperatures. Chinook Salmon tended to be captured at depths and temperatures in-between Brown Trout and the cold, deep water species assemblage. Yellow Perch, Alewife and Round Goby were captured across the broadest range of depths and temperatures (5–30 m; 10–20 °C).
Bottom set gill nets were used to sample the fish community in this study. This raises the possibility of gear and habitat selectivity potentially biasing catches of some species. For example, bottom set gill net may be biased against pelagic species in the fish community, especially in deeper water depths. However, pelagic species often undertake diel vertical movements making them vulnerable to capture at least some of the time during the 24-h gill net set period used in this study. Indeed, one of the most pelagic-oriented fish species caught in this study, Alewife, was the most abundant species caught. Pelagic salmon and trout species, including Chinook Salmon and Rainbow Trout (Oncorhynchus mykiss), may have been relatively under-represented in the bottom set gill nets. However, these species are more important in western Lake Ontario than in the northeastern waters sampled here; highest catches occurred in the western-most areas sampled. Brown Trout appeared to be well represented in the gill net catches and may be more bottom-oriented than Chinook Salmon and Rainbow Trout. A wide-range of gill net mesh sizes was used; nevertheless, small fish species would not be expected to be adequately sampled by this gear type. Rainbow Smelt (Osmerus mordax) was likely a much more important species than the results of the gill net catches indicated. Alewife and Round Goby were generally only captured in the smallest gill net mesh size; only the largest individuals were susceptible to capture. A change in body condition or growth rate could have a major effect on likelihood of capture for these species.
The major increase in Alewife gill net catch after 2004 observed in this study must be interpreted with caution because the mean weight of the average Alewife also increased after 2004. Clearly, there must have been an increase in the abundance of large alewife, those most susceptible to the gill net. But this could be due to increased survival, and hence overall abundance, or an increase in body condition and/or growth. O'Gorman et al. (2008), based on long-term annual bottom trawling in the New York waters of Lake Ontario, found a change point in adult Alewife condition in the fall of 2003. Subsequently, Walsh and Connerton (2013) reported that since 2004, adult Alewife condition in spring and fall was higher than in any other period since the late 1970's but there was no overall increase in abundance of age-2 and older Alewife over 2004–2012 time-period. Based on these results, it seems likely that the increase in Alewife gill net catch in the present study was due to an increase in body condition rather than an overall increase in population abundance. Changes in Alewife diet and behaviour (Stewart et al., 2009; Boscarino et al., 2010) may explain the increased condition in recent years (Holeck et al., 2013).
A major change in fish species composition was the addition of the invasive Round Goby. Round Goby were first caught in the gill nets in 2003 and, by virtue of their unique preference to consume dreissenid mussels, have established a new link in the foodweb from lower trophic levels to piscivorous species (Dietrich et al., 2006). Round Goby were captured at all depths sampled in this study and are now an important and influential member of the entire aquatic foodweb. However, this small species was only vulnerable to the smallest gill net mesh size used here; a large component of the Round Goby population would not be vulnerable to the gill net sampling gear.
Many of the important fish species of northeastern Lake Ontario described in this study showed an abundance decline after the early 1990s (Figure 3). This result could be consistent with an overall decline in primary productivity. Algal biomass, as indicated by chlorophyll a, declined from the mid-1980s through the early 1990s (Millard et al., 2003; Mills et al., 2003). The decline in lower trophic level productivity could have cascaded up to higher trophic levels including fish. Stewart et al. (2010) compared zooplankton production in 1987–1991 and 2001–2005 time-periods in the offshore waters of Lake Ontario. Zooplankton production, driven by declines in cyclopoid copepods, had declined by approximately one-half by the latter time-period in synchrony with declines in primary productivity.
Perhaps the most extraordinary observation in this study is the profound and rapid decline in abundance of the coldwater benthic-oriented fish species assemblage (demersal species dependent on the benthic invertebrate foodweb) after the early 1990s. Specifically, the abundance of the four most important coldwater benthic-oriented species peaked in the early (Lake Trout and Lake Whitefish) or mid 1990s (Burbot and Round Whitefish). Abundance of all four species declined dramatically through the early to mid-2000s before stabilizing at very low levels. So too did the abundance of Slimy Sculpin (Cottus cognatus) (Owens et al., 2003), another benthic fish species but not vulnerable to the gill nets used in this study. These results parallel the decline in Lake Ontario's benthic invertebrate community; sphaeriids, oligochaetes, and especially Diporeia, coincident with the dramatic expansion of Dreissena rostriformis bugensis (Quagga Mussel) (Watkins et al., 2007; Dermott, 2001). The coldwater benthic-oriented fish species, at least at juvenile life-stages (e.g. juvenile Lake Trout, Elrod and O'Gorman, 1991; Lake Whitefish, Hoyle et al., 1999), rely on these benthic invertebrates for food. The Lake Ontario Lake Trout population is supported by stocking. Although stocking cut-backs occurred in the early 1990s, a significant decline in juvenile Lake Trout survival also occurred at that time (Brenden et al., 2011). Hoyle (2005) attributed dramatic changes in Lake Whitefish life history attributes, i.e. slower growth and delayed age-at-maturity, to the loss of Diporiea and the switch in diet to lower-energy prey, primarily dreissenid mussels. Round Whitefish were only captured in the western-most areas sampled in the present study; its distribution being restricted to north central and northwestern Lake Ontario. Less is known about Lake Ontario's Round Whitefish but its abundance may have declined for reasons similar to that for Lake Whitefish. Looking to the future, the loss of the relatively large, energy rich Diporeia in particular, with no indication that Diporeia populations can recover after D. bugensis becomes established, must have serious consequences for on-going benthic fish production (Watkins et al., 2007).
Yellow Perch are currently low in abundance relative to past levels. The reason for this is not clear. Yellow Perch may compete with the newly established Round Goby for food resources. Although dreissenid mussels are the most abundant prey type in the Round Goby diet, Round Goby also commonly consume other prey types, especially chironomids and amphipods (Taraborelli et al., 2010) that may also be important in the Yellow Perch diet (Hurley, 1986b). Double-crested Cormorant (Phalacrocorax auritus) predation on Yellow Perch may be an influencing factor (Burnett et al., 2002). Also, Alewife has the potential to limit Yellow Perch abundance through predation on and/or competition with larval/juvenile life-stages (O'Gorman and Burnett, 2001; Hoyle et al., 2007). Walleye abundance trends were consistent with changes in the Bay of Quinte Walleye population—the largest Lake Ontario spawning stock (Bowlby et al., 2010). Increased recruitment from Lake Ontario proper may account for the slightly increasing trend in overall abundance since 2005. Smallmouth Bass abundance in eastern Lake Ontario is significantly correlated with mid-summer water temperature; warmer summers produce larger year-classes Casselman et al., 2002). Decline in Smallmouth abundance in the 1990s was associated with increased juvenile bass mortality due to Double-crested Cormorant predation (Lantry et al., 2002). Currently, Smallmouth Bass abundance appears to be relatively stable. Growth rate is high and body condition has increased since Round Goby largely replaced crayfish in the Smallmouth Bass diet (Lantry, 2012). Lake Ontario Cisco declined in the 1940s (Christie, 1973), and declined further in the 1990s (Morrison and LaPan, 2007; present study). The decline in the 1940s may be linked to Rainbow Smelt invasion to Lake Ontario but the mechanism of decline in the 1990s is less clear.
Long-term fish community monitoring using bottom set gill nets in a balanced, depth-stratified sampling design effectively described a broad picture of the warm, cool and coldwater fish species assemblages inhabiting open-coastal waters of northeastern Lake Ontario out to about 30 m water depth. While this sampling is geographically isolated to about one-quarter of Lake Ontario (northeast), results may be indicative of large-scale ecosystem-level changes (e.g. primary productivity decline and dreissenid-induced impacts) in Lake Ontario's the aquatic foodweb generally. Many of the dominant fish species showed peak abundance levels in the early 1990s followed by dramatic declines. These declines are coincident with decline in overall ecosystem productivity. The coldwater benthic-oriented fish species assemblage was profoundly affected by the decline in the benthic invertebrate community, especially the loss of Diporeia, which was coincident with the invasion and expansion of dreissenid mussels. The future outlook for this species assemblage appears bleak.
The on-going dedicated efforts of Lake Ontario Fisheries Management Unit staff at the Glenora Fisheries Station are gratefully recognized. Three anonymous reviewers provided significant input on a previous draft of the manuscript.