How to sample juvenile Lake Sturgeon, (Acipenser fulvescens Rafinesque, 1817), in Boreal Shield rivers using gill nets, with an emphasis on assessing recruitment patterns

Abstract

Adaptive management and recovery initiatives for longlived, late-maturing species such as Lake Sturgeon, Acipenser fulvescens, are complicated by temporal lags. By the time anthropogenic impacts on critical periods (spawning, larval hatch, age-0 survival) would be manifested in adult populations, decades might have passed. However, recruitment patterns and population trajectory responses (both positive and negative) can be identified by examining the juvenile life stage. This study describes and evaluates a gill net method for sampling juvenile Lake Sturgeon between 250 and 800 mm fork length (FL) resident in Boreal Shield rivers in relative proportion to their abundance. The method is based on previous observations of deepwater preference (>10-15 m), and employs mesh sizes of 25.4, 50.8, 76.2, 127.0 and 152.4 mm stretched measure. Selectivity curves were generated based on 1040 Lake Sturgeon captures from six reaches of the Winnipeg and Nelson rivers, Canada. A normal (common spread) curve approximated a normal distribution centered on~390 mm FL, and relative selection exceeded 0.65 across the 250-800 mm FL range. For the Slave Falls Reservoir (Winnipeg River), Spearman's rankorder correlation (q) for zone-specific cohort-frequency distributions in adjacent sampling years ranged from 0.85 to 0.93, while the score for the entire reservoir was 0.95, suggesting inter-annual consistency. The method allows for rapid and robust assessments of relative abundance and cohort strength for juvenile Lake Sturgeon within large Boreal Shield river systems, and facilitates biological comparisons among reaches and over time. Incidentally, cohort frequency results derived herein indicate that juvenile recruitment in regulated Boreal Shield rivers can be erratic, irrespective of size of the spawning stock. Such a pattern could be an inherent characteristic of the species that needs to be accounted for when developing adaptive management and species recovery plans. U.S.

Extrinsic Factors Influencing Somatic Growth of Lake Sturgeon

“…Some of these data have previously been examined in other studies published in the primary literature (McDougall et al. , ; Barth and Anderson ), but others have not (Keeyask Hydropower Limited Partnership, unpublished data; Manitoba Hydro, unpublished data; Coordinated Aquatic Monitoring Program, unpublished data).…”

“…Juvenile Lake Sturgeon were captured using methods similar to those described by McDougall et al. (); deepwater habitats were targeted, generally using gill‐net gangs comprised of equal lengths of 25.4‐, 50.8‐, 76.2‐, 127.0‐, and 152.4‐mm stretched‐measure mesh (22.9‐m panels). At a few locations, gangs consisting of 50.8‐, 76.2‐, 95.3‐, 108.0‐, and 127.0‐mm or 25.4‐, 76.2‐, 127.0‐, and 203.2‐mm meshes were used (also 22.9‐m panels).…”

“…In general, the complete first ray of the right pectoral fin ray was removed from the base of the fin articulation, as described by McDougall et al. () and in line with the methods of Parsons et al. (); however, for Winnipeg River sampling conducted between 2006 and 2008, only an ~3‐cm chunk of each fin ray (proximal to the articulation) was removed, as described by Barth and Anderson (), in line with the methods of Nilo et al.…”

“…The impetus for adopting three‐reader consensus methods relates primarily to cohort assignment and the need to quantify reader confidence in the face of aging inaccuracy potentially muting true differences between adjacent strong and weak cohorts (Campana ; McDougall et al. ). Incidentally, McDougall et al.…”

“…Herein, juvenile densities in each of three sections were relatively low (McDougall et al. ). Density dependence therefore seemed implausible, temperature (latitude) differences could again be ruled out as a causal factor, and yet the pattern of growth rates being faster in downstream sections was again observed (data not previously published but presented herein).…”