Misty Lake sticklebacks COSEWIC asessment and status report: chapter 7

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Population sizes and trends

Search effort

No systematic survey to determine distribution of stickleback has been completed in the inlet drainage system. It is likely that stickleback will be found in areas above the known limits shown in Fig. 6, but researchers (Hendry, pers. comm. 2005, 2006; Moore, pers. comm. 2005, 2006) have indicated that they do not occur in the highest reaches of the system.

Abundance

In parapatric lake-stream pairs, lake stickleback are much more numerous than stream stickleback (Hendry and Taylor 2004). To date, no population estimates have been completed for the lake-stream sticklebacks in the Misty Lake system.

Reimchen (1990) estimated an average population in Drizzle Lake (Queen Charlotte Islands, BC) of 75,000 adult sticklebacks based on mark-recapture methods. Drizzle Lake is 112 ha in size; by comparison Misty Lake is 35.6 ha. Moore (pers. comm. 2005, 2006) suggests that the adult population in Misty Lake inlet may be less than 2500 and is likely higher than 4000 in the outlet; these numbers, however, are preliminary estimates based on collection data and may underestimate population sizes.

Fluctuations and trends

It is possible that in drought cycles population declines may occur as a result of habitat loss due to the lowering of the water table; this may be particularly true for the stream form. It is also possible that temporary declines may occur as a result of increased algal growth in the streams following canopy removal due to forest harvesting in the riparian zone (Moore, pers. comm.). However, habitat appears to have remained relatively stable over the longer term and population abundance has likely followed a similar overall trend. There is no other information available to determine trends or whether fluctuations in abundance have occurred in the system.

Rescue effect

Several hundred lakes along the British Columbia coast have been examined for sticklebacks. Although other population pairs have been found (Hendry and Taylor 2004), only three are currently known to contain highly differentiated lake-stream stickleback pairs. At present, extreme divergence of contiguous lake and stream pairs appears to be confined to the northern Vancouver Island area and Graham Island in the Queen Charlotte Islands (Moodie 1972; Reimchen et al. 1985; Lavin and McPhail 1993; McPhail 1994; Taylor, pers. comm. 2005). It seems likely that the adaptive divergence of these pairs requires an uncommon set of conditions and events (Taylor, pers. comm. 2005). If any of the Misty Lake forms are lost, no rescue effect is possible except perhaps between the lake and the outlet, where gene flow is apparent. Although the outlet stream fish are genetically similar to the Misty Lake population, they are not the same ecotype. Furthermore, the inlet stream form is genetically distinct from the lake and outlet populations (Thompson et al. 1997; Hendry et al. 2002). If it is possible for lake-stream forms to re-establish over time, it does not seem likely that each of the new forms would be defined by a preponderance of one of the two divergent ancestral clades as they are today.