Evaluation of a Simple Decompression Procedure to Reduce Decompression Trauma in Trap-Caught Burbot
Matthew D. Neufeld and Colin R. Spence
Burbot Lota lota retrieved from trap depths greater than 10 m often experience decompression trauma as their swim bladders expand and other gases come out of solution during trap ascent (Gitschlag 1986; Bruesewitz et al. 1993). Trauma results from the rapid decline in ambient pressure with decreasing water depth, similar to the “bends” experienced by human divers. Tytler and Blaxter (1973) proposed that safe decompression of Atlantic cod Gadus morhua could be achieved if fish were raised through the water column in discrete steps, exposing fish to pressure reductions of one half the preceding pressure. We tested a variation of their protocol with a simple one-stop decompression procedure to reduce trauma in trap-caught burbot on Duncan Reservoir and Trout Lake, in southeastern British Columbia, Canada. Burbot were captured using cod traps (Spence 2000) typically fished 1–2 d before retrieval, at set depths ranging from 8.0 to 32.0 m. Approximately half of the traps were randomly selected and retrieved following a single stop decompression procedure (Table 1), and half were retrieved from set depth without decompression. A total of 179 trap sets were completed. The depths of decompressed traps and those pulled from depth directly were similar (Mann-Whitney U = 1,946.0, P = 0.370). In total, 89 burbot ranging in total length from 30 to 89 cm were captured. Thirty-five were processed using staged decompression, while 54 were pulled directly from capture depth. To simplify field procedures for decompressed fish, capture depths were grouped into 5-m intervals and a mean decompression stop depth (half capture depth pressure) was calculated for each interval (Table 1). Traps selected for decompression were pulled to the surface to check for the presence of burbot. If one or more burbot were present, the base of the trap was placed in a large tub filled with water to facilitate fish respiration. The trap was then quickly moved to the nearest location with the appropriate decompression stop depth, where the trap was again lowered to the bottom. This procedure usually took less than 30 s and never more than 1 min to complete. Traps were then anchored at the stop depth overnight, for approximately a 24-h period, before being pulled and checked. Upon final retrieval, most burbot were observed for at least 10 min before our observations of decompression trauma were made. Some fish were ultimately transported to a hatchery facility as part of another study, which allowed a better opportunity to assess mortality. In the case of fish held for a number of weeks after trapping, the signs of impending mortality (extremely pale gills and abnormal swimming behavior) occurred within 3–4 min following final trap retrieval. Most mortalities from this group occurred within 10 min of retrieval. Only two mortalities occurred after fish had been transported to the hatchery holding facility, consisting of one fish from each treatment group. Although these fish died more than 2 d after trapping, they were also considered to have succumbed to decompression trauma and were included in data analyses. All burbot captured, regardless of treatment method, exhibited some signs of decompression trauma. Observations of the two most severe symptoms of decompression trauma, mortality and stomach evulsion, were examined because observations not subjective and consistently occurred within 10 min of trap retrieval. When all burbot were compared, the group pulled directly without decompression experienced 22% (n = 12) mortality, while the group treated with decompression procedures experienced 6% (n = 2) mortality—a significant difference (Fisher’s exact p = 0.041). When all burbot were compared, 20% (n = 11) of the group pulled directly from trapping depth experienced stomach evulsion, while the group pulled following decompression procedures experienced no evulsion. Again, a significant difference was observed (Fisher’s exact p = 0.003). Mortalities of decompressed fish did not occur more commonly among the deepest traps with relatively deep decompression stops. Initial trap depths for the two mortalities in the decompression group were 17.6 and 20.3 m, occurring among the shallower set depths tested. In comparison, mortalities in the group pulled from trapping depth directly (n = 12) were spread throughout the range of capture depth (13.3–31.1 m). Although the method tested did not prevent decompression trauma, the technique resulted in significantly fewer mortalities and lower occurrence of stomach evulsion. The methods were developed to deal with studies where prolonged exposure of burbot to low pressure conditions is expected following trapping. In the present study, for example, many of the fish were destined for radio tagging or shallow tubs in a hatchery. However, where simple objectives such sampling and tagging are involved, quick handling (<1 min) combined with the use of a cage to lower fish back to depth may be more appropriate.