State of the Greater Fundy Ecosystem - Genetic variation in brook trout

GREATER FUNDY ECOSYSTEM RESEARCH PROJECT

UNB Faculty of Forestry and Environmental Management

State of the Greater Fundy Ecosystem

Low Levels of Genetic Variation in
Brook Trout Lake Populations
in Fundy National Park:
Preliminary Results

Matthew Jones1, Tara McParland1,2, Jeffrey Hutchings1,
Roy Danzmann2 and Douglas Clay3
1 Dept. of Biology, Dalhousie University, Halifax, N.S., B3H 4J1
2Dept. of Zoology, University of Guelph, Guelph, ON., N1G 2W1
3Parks Canada, Fundy National Park, Alma, N.B., E0A 1B0

This study examined the amount of genetic variation in Brook Trout lake and stream populations and the extent of movement between one lake and its tributaries. The amount of genetic variation present in a population is important to its long term sustainability. In a previous study of genetic variation in Brook Trout throughout the Maritimes, Jones et al. (1996) found the population heterozygosity levels (a metric of genetic variation) in three lake populations to be the lowest of all populations examined. The heterozygosity levels of the three lakes (Bennett, Tracey and Wolfe) all in the Point Wolfe River drainage, were approximately half that of three stream populations examined from this drainage (Figure 1). This finding was unexpected. The lower levels of heterozygosity suggested a lower effective population size for the lakes as compared to the streams. A population's effective size often differs from its actual population size. Effective population size is a measure of the number of reproducing individuals and is affected by such factors as the sex ratio, the number of founders, population bottlenecks, overlapping generations and individual variation in reproductive success. High mortality from environmental factors, angling or predation and/or limited spawning grounds could account for the lower heterozygosities observed for the lakes. However, at least for Bennett and Tracey lakes, immigration of Brook Trout from inlet streams might be expected to reduce the effect of such events. The extent of migration between streams and lakes in Fundy National Park (Fundy NP) and the factors which may be associated with the low heterozygosity levels are unknown.

Figure 1. Observed (Ho) and expected (He) heterozygostities (based on 34 loci)
for the Point Wolfe River populations examined in Jones et al. (1996). Number in
observed heterozygosity column is the sample size; bars represent standard errors.

GOALS

The objectives of this study were:

1) To examine Brook Trout movements between Bennett Lake and its inlet streams.
2) To quantify Brook Trout population size and heterozygosity levels for Bennett Lake and its inlet streams.
3) To determine Brook Trout population heterozygosity levels for lakes of differing sizes and angling intensities in the GFE.

METHODS

Movement between habitats in Bennett Lake

Bennett Lake was chosen to be the focal lake of this study because of its easy access, high angling pressure, the presence of three inlet streams, and because it is the lake in Fundy NP with the largest body of historical Brook Trout data. Prior to and following the fishing season (i.e. before Victoria Day weekend and after Labour Day weekend) in 1996 and 1997, Brook Trout were sampled from Bennett Lake using fyke nets and box traps. The nets were moved after each sampling interval in the fall. A small 2-way fish fence was installed in Unnamed Brook 25m upstream from Bennett Lake in early fall 1996. The three tributaries to Bennett Lake were sampled by electrofishing. The fish were group or individually marked. Each fish's reproductive state was determined during the fall surveys. These surveys provided estimates on the number of fish present in each habitat as well as the number of reproducing individuals in each habitat.

Heterozygosity Levels

The Brook Trout lake populations surveyed in Fundy NP for genetic variation had half the variation of the 30 stream populations sampled throughout the Maritimes and half the genetic variation of three streams in the same river drainage (i.e. the Point Wolfe River drainage). However, no examination was made of adjacent lake and riverine habitats. It was, therefore, necessary to first test the hypothesis that there is a "lake effect" regarding levels of genetic variation. This was tested by comparing the levels of genetic variation in the Brook Trout populations in Bennett Lake and its three inlet tributaries. Brook Trout were later collected for other lakes and associated streams (Wood and Pleasant lakes remain to be completed). Collections of Brook Trout for genetic analysis were made using electrofishing (for the brooks) and fyke nets, gill nets and angling (for the lakes). Protein electrophoresis was carried out as described by Jones et al. (1996).

RESULTS

Movement

Only one fish marked in a stream was recaptured in Bennett Lake . This fish had been tagged June 5 in the lower 100 m of Unnamed Brook and was recaptured approximately 200 m from the brook on October 29th. Little movement through the fish fence on Unnamed Brook was apparent until October 19th. Several of the mature, ripe fish caught in the upstream trap in Unnamed Brook were marked indicating they were previously in Bennett Lake. Spent fish were found in the downstream trap. Large mature fish were only found in the lower 100 m (to a digger log -an approximately 25 cm barrier) when the lower 500 m of this brook was electrofished November 8th. It would appear lake resident fish use the lower section of the brook for spawning. No lake resident fish were found in Tracey Lake Brook during the same time period, however, only a small section was sampled. In June 1997, two fish marked earlier in the spring in Bennett Lake were found 300 m upstream in Unnamed Brook.

Within habitats, evidence of movement exists. In Bennett Lake during individual sampling periods, recaptures of the individually marked fish were often hundreds of meters from their original capture location with some recaptured at the opposite end of the lake (mean 377 m, std 238; n=94). Between sampling periods, movement appears to be more extensive (mean 532 m, std 266; n=15). Less sampling has to date been conducted in the streams, however preliminary results indicate some movement with some fish recaptured over 100 m from their original capture location.

Population size

Mark-recapture experiments were conducted in Bennett Lake in the fall of 1996 and spring 1997. The fall estimate was 7111 trout over 10 cm (6324-8056 95% C.I. based on 1852 marked and 262 recaptures). The spring estimate was 11286 trout over 10 cm (8752-15131 95% C.I. based on 1197 marked and 51 recaptures). The spring estimate includes age 1+ fish which were smaller than 10 cm the previous fall. The majority of the trout captured in the fall experiment were in spawning condition. Similar mark-recapture experiments are being made for the stream populations, however, more data is required for accurate estimates.

Heterozygosity

The heterozygosity level of the three Bennett Lake tributary populations were all greater than that of the Bennett Lake population heterozygosity level (Figure 2). The levels of Kelly and Tracey Lake Brook populations were approximately 60% greater than that of the Bennett Lake population, while Unnamed Brook population heterozygosity was approximately 10% greater.

Figure 2. Observed (Ho) and expected (He) heterozygosities (based on the seven
variable loci) for Bennett Lake and its three tributaries. Number in observed heterozygosity
column is the sample size; bars represent standard errors.

Two of the four lake populations sampled for this study had similar or higher heterozygosity levels than their associated stream population (Dick's and Laverty Lakes respectively; Figure 3). All Point Wolfe River populations (Wolfe, Tracey and Bennett lakes and East Branch, Unnamed and Tracey Lake brooks) had lower heterozygosity levels than all other populations sampled in this study.

Figure 3. Expected heterozygosities (based on the seven variable loci) for lake
and associated stream populations. Bars represent standard errors. Laverty Brook,
Wolfe Lake, Tracey Lake and East Branch data from Hones et al. (1996)

IMPLICATIONS FOR MANAGEMENT

Bennett Lake has lower levels of heterozygosity than its inlet tributaries despite currently having a large number of reproducing fish. There are negative implications of the lower heterozygosities in the Fundy NP lakes. Some studies have suggested a positive association between heterozygosity and components of fitness (ability to survive and reproduce) in salmonids. Similarly, decreases in heterozygosity in salmonid hatchery strains are thought to have a negative influence on such factors as survival, growth and development stability. Furthermore, lower genetic variation may render a population incapable of adapting to changing environmental conditions. The finding that not all lakes have lower heterozygosity levels than streams in the same drainage suggests that lake populations do not inherently have lower levels of genetic variation than stream populations. The number of spawning fish present in the fall in Bennett Lake suggests that current angling intensity is not adversely affecting the population.

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