Origin and Abundance of Southern Gulf of St. Lawrence
Striped Bass: New Perspectives

Rod Bradford

Background

The anadromous striped bass (Morone saxatilis Order Perciformes; Family Percichthydae) has a natural range that extends from the Gulf of Mexico in the south northward to the St. Lawrence River estuary (Scott and Scott 1988). The Chesapeake Bay region of the eastern United States of America is generally regarded to be the center of their distribution and the region of highest abundance. There are two sites within the Maritimes Region where striped bass are known to spawn on a year to year basis, the Northwest Miramichi Estuary, southern Gulf of St. Lawrence, and the Shubenacadie River, inner Bay of Fundy (Figure 1). The two populations are genetically discrete (Wirgin et al. 1993) and both differ genetically from the Hudson River and Chesapeake Bay stocks (Wirgin et al. 1993).

Adult striped bass enter freshwater to spawn during the vernal spring. The upstream migration in the eastern Canadian rivers occurs during May-June and rarely extends far above the head of tide. Individual fish usually return to the sea within two weeks. Spawning appears to be triggered when water temperatures climb to 16-18 ⁰C (Robichaud-LeBlanc et al. 1996). Eggs and milt are released directly into the water column where the fertilized eggs, being semi-buoyant, remain suspended for the duration of the incubation period. Free swimming, pelagic yolk-sac larvae hatch out after about 72 h. Yolk reserves are exhausted within about seven to 10 days after which the larvae forage on zooplankton within the estuary (Robichaud-LeBlanc et al. 1997). The transition from larva to juvenile occurs within about six weeks after which the young fish form schools and assume a more demersal mode of existence. Adult and juvenile striped bass can range several hundred kilometers beyond their natal rivers during the summer and autumn months, usually remaining within a few kilometers of the shore. Males reach maturity at about age three whereas females generally spawn for the first time at age four (Bradford and Chaput 1996).

Figure 1. Map of Maritimes Region showing the location of place names used in the text and a detailed representation of the Northwest Miramichi River Estuary.

Striped bass is a highly prized and economically valuable game fish along the eastern seaboard of the United States. Recreational angling fisheries for striped bass in eastern Canadian waters have remained poorly developed, a reflection in part of the uncertainty of the supply of fish available to capture from year to year. Trends in reported landings indicate that striped bass native to eastern Canada rivers have been prone to cycles of 'boom and bust'. The underlying causes of these cycles were never clearly elucidated, although environmental factors and fishing mortality were both considered to be contributing factors (Chaput and Randall 1990).

Figure 2. Length frequency distributions of adult striped bass on the Northwest Miramichi River Estuary for the years 1993 to 1997. Also shown are the sample sizes, estimates of spawner abundance, and the number of females represented in the population (YC =yearclass)

Figure 3. Catch per unit effort (fish per net per 24 h period) of young of the year striped bass in the open water smelt fishery of the Miramichi River at upstream and downstream locations, 1991 to 1997. Solid line is the annual geometric mean at the downstream sampling location. Individual points may represent several observations.

Conservation Management Plan

Concern for the long term viability of the resource arising from the absence of formal resource management and the suspected negative consequences of unregulated fishing (Chaput and Randall 1990) led to the development of a conservation management plan in 1993 (Dept. Of Fisheries and Oceans 1993).

* Arrest the decline in abundance increase abundance
* And sustain abundance at levels correspondent to supporting habitat.

The plan was fully implemented in March, 1996: sales of Canadian, wild-caught, striped bass are now illegal. Management is now in support of the development of recreational angling fisheries, which are anticipated to generate economic activity in excess of $10 million annually within New Brunswick alone (Loftus et al. 1993). Striped bass continue to serve the traditional food, social, and ceremonial needs of native Americans throughout the region (Perley 1852; Bradford and Chaput 1996).

A research program in support of the conservation management plan and recreational fisheries development was launched in the spring of 1993. The principle goals were to establish the protocols for calculating both adult abundance and recruitment, as well as the stock structure, migration patterns, and habitat requirements of striped bass in the southern Gulf of St. Lawrence (Figure 1). The major elements of this program and the results to date are summarized below.

Southern Gulf of St. Lawrence Striped Bass Project

The southern Gulf of St. Lawrence (southern Gulf) is the center of striped bass production within the Maritime Provinces (Chaput and Randall 1990). There, striped bass have been fished commercially since before Canadian Confederation (Perley 1857) with reported landings exceeding 40 t in some years (LeBlanc and Chaput 1991).

Interestingly, relatively little fishing effort has ever directly targeted striped bass. Instead, fish intercepted as a bycatch in the inshore commercial fisheries were landed and sold (LeBlanc and Chaput 1991; Bradford et al. 1995). Early results from the research program indicated that striped bass bycatch from the gaspereau fishery during May-June could provide a basis for assessments of adult abundance and that the bycatch from the smelt fisheries during October-November could provide an indication of the abundance of juveniles produced from the spawning of the previous spring (Bradford et al. 1995). Furthermore, the gear types used in both fisheries hold the captured fish live until the nets are fished and the catch loaded onto the fishing vessel. Thus, striped bass can be sorted from the catch, sampled, and returned live to the water.
The Northwest Miramichi Estuary (Figure 1) was identified as the most likely site within the southern Gulf for obtaining reliable annual estimates of adult abundance while the fish were concentrated on the spawning grounds. First, interannually persistent high levels of adult bycatch in the gaspereau fishery during May-June were evident in the time series of reported landings of striped bass from the commercial fishery (LeBlanc and Chaput 1991). Second, biological sampling of the bycatch on the Northwest Miramichi Estuary demonstrated that the fish were in spawning condition (Bradford et al. 1995). Third, striped bass eggs and larvae are members of the planktonic community within the estuary at the same time that reproductively mature striped bass dominate the bycatch in the gaspereau fishery (Bradford et al. 1995; Robichaud-LeBlanc et al. 1996).

Mark-recapture experiments are used to estimate adult abundance. A sample population of adult fish are marked (with external, individually numbered tags inserted into the anterior dorsal fin) about one to two weeks prior to spawning. Each fish is also measured, examined externally to determine its sex, and a scale sample is extracted for age and growth studies. During the spawning season, the number of adult striped bass caught in the gaspereau traps are counted and the total number of recaptures noted. Thus by knowing the number of tags available, the number of recaptures, and the total number of striped bass in the sampled catch, one can estimate the total number of adult striped bass. Furthermore, the tag numbers for all recaptured fish are recorded before the fish are returned to the wild. Over a period of several years these data can resolve seasonal migrations, whether or not individual fish spawn every year, and the degree of fidelity that individual fish exhibit to the Miramichi estuary as a spawning location.

Figure 4. Length frequency distributions of young of the year striped bass in the open water smelt fishery of the Miramichi River for all locations, 1991 to 1996. The modal length is shaded (n =sample size

Range and Spawning Site Affinity

The known range of the Miramichi population encompasses the entire mainland portions of the southern Gulf of St. Lawrence. Recaptures have been reported from Percé, Québec and fish marked at DFO Science Traps on the Margaree, Nova Scotia (Figure 1) have been recaptured as spawners on the Northwest Miramichi. Multiple year recaptures indicate that Miramichi striped bass spawn every year from age 3 through to age seven, and all return to Northwest Miramichi estuary in subsequent years to spawn.

Spawner Abundance

Adult abundance estimates for the years 1993 to 1997 are shown on Figure 2 along with the corresponding length frequency distribution of the catch for that year. The dramatic 80% decline in adult abundance between the years 1995 and 1996 was nearly completely a consequence of the unregulated commercial fishing that occurred during this time (Bradford and Chaput 1997). This demonstrates how unregulated fishing has contributed to the characteristic cycle of 'boom and bust' for this population. Inspection of the adult size distributions for each year shows that the 1991 yearclass that first recruited at age 3 (males only) in 1994 has remained the dominant component of the spawning population through to 1997 (Figure 2; fish smaller than ~32 cm Fork Length are age 2⁺ and immature). One can also note that a yearclass failure occurred in 1993 with the result that recruitment to the adult population was negligible both by males at age 3 in 1996, and by females at age 4 in 1997 (Figure 2). This can be partially explained by the fact that <350 females spawned during the spring of 1993 (Figure 2) with the result that the production of young of the year was correspondingly low (Figure 3). In comparison, the highest juvenile abundance from the autumn smelt fishery occurred during the years when female abundance was higher than 5000 fish (years 1995 and 1996; Figure 3). However, studies on size at age indicate that additional factors can significantly influence recruitment, as explained below.

Figure 5. Length frequency histograms and cumulative frequency distibution (line and symbol) of observed prewinter lengths of young of the year striped bass on the Miramichi River, 1991 to 1993. The solid line with no symbol shows the cumulative length frequency of survivors to age 1 as estimated by back-calculation of length at age from scale annuli.

Environmental/Physiological Constraints on Juvenile Production

The Miramichi estuary represents the northern limit of spawning for striped bass (Bradford et al. 1995a; Rulifson and Dadswell 1995). Like most fishes that reproduce at the geographic extremes of their natural range, southern Gulf striped bass are highly susceptible to the vagaries of environmental factors that can influence growth. A striking feature of the juvenile populations of striped bass sampled from the southern Gulf is the variability among years in body size at the end of the first growth season. Figure 4 shows that median length can vary by up to three centimeters among years. It can also be noted that the strong 1991 yearclass although not highly abundant when compared to the other age classes was composed of relatively large bodied individuals (Figure 4). These observations indicate that there is a size-dependent component to survivorship for northern populations of striped bass.

Comparisons of the observed juvenile pre-winter size distributions (Age 0⁺) for the 1991, 1992, and 1993 yearclasses (the bycatch from the autumn Miramichi smelt fishery), with the length frequency of survivors to age 1 (estimated by back-calculating to length at age 1 from the annuli recorded in the body scales of the fish) shows (Figure 5) that striped bass need to be larger than about 10 cm Fork Length at the end of their first growth season in order for there to be a reasonable chance for survivorship through the first winter. The underlying mechanisms are not fully resolved at this time but seem to be a consequence of the fact that southern Gulf striped bass do not feed when water temperatures drop below 4 ⁰C. Instead, energy stored as fats are utilized by the fish to sustain their metabolic obligations throughout the ensuing 4 months of winter. Under similar circumstances elsewhere and for other species of fish it has been shown that larger bodied fish have greater starvation endurance, and perhaps lower osmotic stress, and therefore higher survival through the first winter (Shuter and Post 1990).

Figure 7. Cumulative ogives of depth averaged temperature and depth average salinity for all hydrographic stations sampled on the St. Louis River, January-March 1997 and for stations where striped bass implanted with ultrasonic transmitters were located.

Figure 6. Spatial distribution (shaded portion) of striped bass tracked under the ice on the St. Louis River by day of sampling for the month of February, 1997. Numbers in parentheses represent the number of striped bass implanted with ultrasonic transmitters that were located on the day of sampling. The 1 km scale bar is shown

Winter Habitat

Within the southern Gulf, striped bass return to freshwater habitats at the onset of winter conditions (Hogans and Melvin 1984; Hanson and Courtenay 1995; Rulifson and Dadswell 1995; Bradford and Chaput 1996), presumably to avoid lethal low marine temperatures. Site selection for overwintering appears to be opportunistic, and largely dependent on their geographic location at the onset of winter (Bradford and Chaput 1996), such that Miramichi-spawning fish do not necessarily return to the Miramichi before winter (Bradford et al. 1995a; Bradford and Chaput 1996).

Virtually all of the major rivers flowing to the southern Gulf of St. Lawrence harbour wintering populations of striped bass. This fact has been common knowledge throughout the region for well over a century (Perley 1852), and not surprisingly the winter aggregations of striped bass were fished for food and for income through to ice-out the following spring. The association between the hydrographic conditions in the estuary and the actual distribution of the wintering fish was not well understood and without this information it was impossible to determine if the sites where winter fishing occurred reflected the limits of the fishes winter distribution or simply sites where the fish were more susceptible to capture. Although commercial fishing is no longer allowed, this kind of knowledge has contemporary value in offering a basis to fine tune protective measures against poaching, and in assessments of the consequences of land- and sea-use practices on habitat viability. In order to address these issues 10 striped bass were implanted with ultrasonic transmitters and tracked under the ice on the Kouchibouguac and St. Louis Rivers of southeastern New Brunswick during the winter of 1996-1997.

The results offer a compelling analogy to the proverbial 'fish in a barrel'. The tracked fish were restricted to the less than 1 km of estuary that corresponded to the historical distributions of the winter fisheries. Hydrographic profiles of the estuary showed the fish to be aggregated within the salt-wedge of the estuaries (where the fresh water discharge from the rivers met the sea). The February distribution of striped bass on the St. Louis River is shown in Figure 6 by way of example. Figure 7 demonstrates the affinity of the wintering striped bass with the salt-wedge throughout the winter months.

Conclusions and Further Studies

The cumulative results to date from the continuing studies on the southern Gulf of St. Lawrence striped bass offer a rather stark reminder of how human activities, in the absence of a resource management framework, can detrimentally affect the natural cycle of fish populations. The constraints on removals of both juvenile and adult striped bass embedded within the conservation management plan must be viewed as a positive development for the long term. Natural factors are now more likely to define the dynamics of this population, and for the first time since Canadian Confederation. The extent to which human land- and sea-use practices impact on striped bass spawning, rearing, and wintering habitat will, however, bear further consideration.

The stock structure of the southern Gulf of St. Lawrence striped bass remains, perhaps, the most important unresolved component of the current resource management effort. The weight of accumulated evidence indicates that these fish comprise a single biological unit that spawns exclusively on the Northwest Miramichi Estuary. Eggs and larvae have not been found outside of the Miramichi River Estuary (Robinson et al. 1997; Bradford and Chaput 1998) and the mark-recapture data indicates that fish known to have spawned at least once on the Miramichi show no tendency to spawn elsewhere in the southern Gulf in subsequent years (R.G. Bradford, unpublished data). However, there is the possibility that the unregulated fishing practices of the past resulted in the extirpation of striped bass populations elsewhere in the southern Gulf. Extension of spawning activity in future years to estuaries beyond the Miramichi River cannot be discounted. If this were to occur the continued management of southern Gulf striped bass as a single biological unit would come into question. Thus, a means to identify quickly when a shift in spawning distribution has occurred would have tremendous value as a resource management tool. The usefulness of the mark-recapture experiment as a means to monitor change in the spawning distribution of striped bass in the southern Gulf is currently under evaluation.

Past and Current Partners

· Burnt Church First Nation · Eel Ground First Nation · Kouchibouguac National Park · Miramichi River Environmental Assessment Committee · Miramichi Watershed Management Committee · New Brunswick Wildlife Federation · Southeast Anglers Association

Literature Cited

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