Michael G. Fox

Growth, Density, and Interspecific Influences on Pumpkinseed Sunfish Life Histories

I collected data on 27 pumpkinseed (Lepomis gibbosus) populations in east- ern and central Ontario and conducted a transplant experiment with one of these popu- lations to test growth and competition-related predictions of several life history models. The predictions are that early maturity and high gonadal investment will occur: (1) in large and fast-growing juveniles; (2) in populations with low adult: juvenile growth ratios; (3) in populations with low density; and (4) in the absence of bluegill sunfish (Lepomis mac- rochirus), a competing congener. Predictions 1 and 4 were fully supported and Prediction 2 was partly supported, but Prediction 3 was not. The earliest maturing juveniles within populations were significantly larger than conspecifics of the same age that did not mature. Females from a lake population that were transplanted into a fishless pond exhibited both significantly faster growth and a significantly higher gonad to body mass ratio (gonado- somatic index) than females living in the lake. At the population level, juvenile growth was significantly correlated with age at maturity (r = -0.53), but not with gonadosomatic index, whereas the Adult: Juvenile Growth Ratio showed a significant, negative correlation with gonadosomatic index (r = -0.66), but not with mean age at maturity. Mean age at maturity showed a stronger correlation with the indicator of life-span and adult survival rate (r = 0.74), suggesting that mortality is more important than growth in the shaping of reproductive life histories of pumpkinseed. High population density was associated with early maturity and high gonadal invest- ment, the opposite of what was predicted. However, pumpkinseed populations that co- occurred with bluegills did mature significantly later and at a larger size and tended to have a lower gonadal investment than populations living in waterbodies without bluegills. This difference in reproductive patterns in the presence of bluegill is consistent with two-stage life history theory and may be the result of a direct effect of bluegills on pumpkinseed growth and survivorship, as well as differences in environmental conditions where the two species do or do not co-occur.

Growth and life history traits of introduced pumpkinseed (Lepomis gibbosus) in Europe, and the relevance to its potential invasiveness

The North American centrarchid, pumpkinseed Lepomis gibbosus (Linnaeus), is a good example of how the life history traits and biogeographical patterns of growth can vary in populations of introduced species, facilitating the establishment of new populations in novel environments. Introduced into Europe from North America in the late 19th century (Maes 1898), the pumpkinseed is now established in at least 28 countries of Europe and Asia Minor (Fig. 1). Scientific interest in the species in Europe began early (e.g. Kunstler 1908, Roule 1931), but studies of the species (i.e. papers with either the Latin or common names, pumpkinseed, or sunfish, in the title) only really began in the mid-20th century (Fig. 2). Scientific output rose to about one publication per year in the 1970s, though half of these were laboratory experiments on

Round goby (Neogobius melanostomus) population structure, biomass, prey consumption and mortality from predation in the Bay of Quinte, Lake Ontario

ABSTRACT We compared round goby (Neogobius melanostomus) population structure and biomass, estimated the quantity of prey consumed by round goby and assessed predation on round goby in two areas of the Bay of Quinte (upper and lower bay) that were invaded by this species two years apart. A bioenergetics approach was used to estimate round goby consumption and to illustrate the influence of changes in the round goby biomass, from May to October in both areas of the bay. Mean round goby biomass was 5 tonnes/km2 for the upper bay and 11.2 tonnes/km2 for the lower bay. Round goby populations were dominated by age-0 to age-2 individuals; older individuals were rare although they constituted a greater portion of total biomass in the earlier invaded lower bay than the upper bay. Estimated round goby consumption was 78.4 tonnes/km2 for the upper bay and 127.1 tonnes/km2 for the lower bay; dreissenids were the most important prey followed by chironomids, Zooplankton and amphipods. Diet analysis showed that round gobies were consumed by all predator species analyzed, and constituted a high percentage of stomach content biomass in yellow perch (Perca flavescens), largemouth bass (Micropterus salmoides) and northern pike (Esox lucius). Age structure differences between the upper and lower bay are consistent with those expected between expanding and stable populations.

Occupation, body size and sex ratio of round goby (Neogobius melanostomus) in established and newly invaded areas of an Ontario river

Invasive species represent a challenge because the particular characteristics of a species’ invasion are often unknown before the invasion. To provide some clarity as to how invasive species demographic structure might change as a population advances its range, we compared the proportion of occupied sites, size structure and sex ratio of round gobies in the area where they first invaded with more recently invaded areas at the extent of their range in a river in south-eastern Ontario. We used a standardized angling technique to sample gobies larger than 45-mm total length in the summer and early autumn of 2007. Round goby at the upstream and downstream extent of their range occupied a lower proportion of randomly selected sites, and contained a wider distribution of sizes as well as significantly larger individuals. Sex ratios in all areas were male-biased and the male-to-female ratio was significantly higher in the upstream segment of the river (2.2:1) compared to the area of first introduction (1.4:1). The difference between the newly invaded and the established sites suggests that round goby population structure may be affected by density. The results of this study help us further describe the demographic characteristics of biological invasions whilst examples of population structure and behaviour in gobies and other species provide a basis for generating hypotheses for range expansion.

Influence of Predation Risk on the Overwinter Mortality and Energetic Relationships of Young-of-Year Walleyes

Abstract We investigated three potential mechanisms—size-dependent overwinter energy loss, size-specific predation, and size-dependent predator-induced metabolic costs—to determine the role of body size and predators on the overwinter survival and energy reserves of young-of-the-year walleyes Stizostedion vitreum. Walleyes were reared in outdoor hatchery ponds in Westport, Ontario, for three overwinter periods (1997-1998, 1998-1999, 1999-2000) in the presence or absence of predators (burbot Lota lota or adult walleyes). Other young-of-the-year fishes were provided as potential prey items. We predicted that (1) smaller individuals of a young-of-the-year walleye cohort would have lower prewinter energy stocks and would deplete them at a higher rate; (2) no size-dependent overwinter survival would occur in walleyes reared without predators; and (3) physiological stress, in combination with size-selective predation on smaller individuals, would lead to size-dependent overwinter mortality in young of the year ...

Range expansion dynamics of the invasive round goby (Neogobius melanostomus) in a river system

The rates of upstream and downstream range expansion of the round goby (Neogobius melanostomus) were examined in the Trent-Severn Waterway in 2009 and 2010. Relative abundance, demographics, and habitat use were also compared between areas of range expansion and a longer established area to determine how these characteristics varied along the invasion pathway. Round gobies were sampled using an angling removal method in May and August of 2009 and 2010 at 75 sites at each area of range expansion and 25 sites near the center of their range where they first became established in the waterway. Areas of range expansion had initially low abundance and low site occupancy in May 2009 relative to the longer established area. Large increases in abundance and site occupancy were observed over the first summer of occupation, but with limited range expansion. Rapid range expansion was observed during the non-reproductive season at the upstream edge of range. Individuals sampled in the expanded upstream range were small and female biased relative to other range locations. Round gobies also exhibited higher habitat selectivity for rocky substrates at range edges than in the longer established area. These characteristics of seasonal dispersal, biased demographics of dispersers, and high habitat selectivity may be important components of successful range expansion of the round goby in invaded ecosystems.

THE INFLUENCE OF AGE-SPECIFIC SURVIVORSHIP ON PUMPKINSEED SUNFISH LIFE HISTORIES

Data from five pumpkinseed sunfish (Lepomis gibbosus) populations were used to generate age-specific survival rates and to test the prediction of life history theory that early maturity and high annual reproductive effort are favored by low adult survival relative to that of juveniles. The prediction relating age-specific survival to reproductive effort was supported by a strong negative correlation (r = -0.95) between the ratio of adult-to-juvenile survivorship and the mean gonadosomatic index of females in the pop- ulation. The relationship between the adult-to-juvenile survival ratio and the mean age at maturity was in the direction predicted by life history theory, but it was not significant. Strong correlations between mean age at maturity and both juvenile growth and the adult- to-juvenile growth ratio indicate that growth may affect the timing of maturity more strongly than survivorship in some species. We estimated the survival cost of reproduction in the populations with the use of demographic data in an optimal life history model developed from the discrete version of the Euler-Lotka equation. Estimated costs ranged from 5% to 52%, with the two populations exhibiting early maturity, high reproductive allocation, and stunted adult body size dis- playing the highest survival costs of reproduction in the model. Annual survivorship curves from adults in these two populations showed a marked concavity not illustrated in the other populations, supporting the prediction of life history theory that high reproductive effort results in a high survival cost of reproduction. The selective pressure of a low adult-to- juvenile survival ratio appears to favor the evolution of increased reproductive effort in pumpkinseeds, even when this results in the further decline of an already low adult survival rate.

Habitat and home range fidelity in a trophically dimorphic pumpkinseed sunfish (Lepomis gibbosus) population

We investigated habitat selection in a trophically dimorphic population of pumpkinseed sunfish (Lepomis gibbosus) to determine whether littoral and limnetic ecotypes exhibit habitat or site fidelity. A transplant experiment was conducted, in which 998 pumpkinseeds captured from littoral and limnetic sites were marked and released in either the site of capture, the nearest site of the same habitat type, or the nearest site of the opposite habitat type. Daily recapture attempts over the course of the reproductive and growing season provided a 25% recapture rate, 40% of which were recaptured on multiple occasions at the same site. Site fidelity was very high in both ecotypes. Results estimated with a multi-state transition model indicated that the probability of a transplanted pumpkinseed returning to its site of origin ranged from 74% for limnetic pumpkinseeds released into a different limnetic or littoral site, to 93% for littoral pumpkinseeds released into a limnetic site. Furthermore, the probability of a pumpkinseed being recaptured at its site of origin if not transplanted was estimated at 97 and 98% for limnetic individuals and littoral individuals, respectively. Discriminant Function Analysis of helminth parasite loads sampled from littoral and limnetic individuals could classify site of origin with 96–100% accuracy, suggesting that the habitat and site fidelity patterns observed with mark–recapture are indicative of long-term habitat segregation of the two forms. The results of our experiment provide compelling evidence of correlated habitat selection as a function of home range fidelity within both ecotypes of a subtly dimorphic species. Such behaviour could have a significant effect on present or future gene flow.

Age and size-dependent variation in the seasonal timing and probability of reproduction among mature female pumpkinseed,Lepomis gibbosus

SynopsisIn this study, we used pumpkinseed,Lepomis gibbosus, populations in two east-central Ontario lakes to test for age and size-dependent effects on the probability and timing of reproduction of mature females. Pumpkinseed body size characteristics differed in the two lakes; Little Round Lake harbours a stunted population and Beloporine Lake does not. Age-class distribution of mature females was determined by a maturity assessment on fish collected just prior to spawning and at the mid-spawning period, combined with an early-season capture-mark-recapture survey. Spawning females were collected throughout the breeding season to assess age and size-related temporal trends, and to compare their age and size distribution with that of mature females at large. The proportion of age 2 females in Little Round Lake that actually spawned was considerably lower than the proportion of mature age 2 females at large. Furthermore, age 2 females that spawned in this lake did not do so until late in the breeding season. In contrast, the proportion of young/small females spawning in Beloporine Lake was comparable to the proportion of young/small mature females at large, and both small and large females spawned throughout the breeding season. Small mature females in Little Round Lake may have had no other option but to spawn late in the season because of their poor body condition. In Beloporine Lake, condition factor early in the breeding season in age 2–4 females was higher than that of Little Round Lake females, suggesting that limited energy reserves in the spring may have prevented young Little Round Lake females from early spawning. Our results show that the likelihood and timing of reproduction are both age and size-dependent in some populations. Small individuals that delay seasonal maturation and spawn late in the summer probably contribute little to the population due to the restricted growth and reduced overwinter survival of their progeny.

Is there a role for aggression in round goby invasion fronts

The role of aggression as a factor promoting invasiveness remains hotly debated. Increased aggression or a lack of tolerance for conspecifics may promote population spread. Some previous research suggests that more aggressive or bold individuals are increasingly likely to disperse and as such these individuals may be overrepresented at the invasion front. In contrast, it has also been argued that individuals at the invasion front represent the least aggressive or least competitive individuals in the population, as these animals are excluded from established areas. Accordingly, the invasion front should be made up of shy, submissive individuals that exhibit reduced aggression. In this study we explore these alternative predictions by quantifying the levels of intra-specific aggression in the round goby (Neogobius melanostomus), an invasive fish that continues to spread rapidly through the Laurentian Great Lakes region in North America. We collected size matched male round goby from an invasion front as well as from an area with an established population, and we staged resource contests between them. Invasion front fish won 65% of the contests and tended to perform more aggressive acts overall. Invasion front fish were not more active or bold prior to the contest, and used the same types of aggressive displays as fish from established areas. Our results also showed that body size asymmetry was an overriding determinant of competitive outcomes, and that body size rather than individual variation in aggressiveness might be the most important contributing factor determining the composition of round goby invasion fronts throughout the Laurentian Great Lakes and its tributaries.