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According to the Cambridge English dictionary, the very definition of the word ‘maritime’ means “near the coast”; and for Dr. Daniel Ruzzante and Greg McCracken, their work at Dalhousie University in Halifax, Nova Scotia, and their passion for science led them straight to the water. Dr. Ruzzante, a professor in the Department of Biology at Dalhousie, is mainly interested in evolutionary and conservation biology as it relates to biodiversity and works in both marine and freshwater systems. His research has focused on many facets of the population structures of different fish species in different ecosystems—one venture of his in particular, honing in on population estimates of brook trout (Salvelinus fentinalis). Conducted in collaboration with John MacMillan of the Inland Fisheries Division of the Nova Scotia Department of Fisheries and Aquaculture, this study was completed with the goal of evaluating novel methodology used in the assessment of such fish populations. Greg McCracken, a Research Associate and Lab Manager also working at Dalhousie, is involved in this project as well.
“Our lab is currently working with a new population estimation method [close-kin mark-recapture] to estimate census population size or abundance from estimates of the number of related individuals found in a relatively small subset of the population.” - Mr. McCracken
According to resources provided by the Department of Fisheries and Oceans, the rivers and streams of Atlantic Canada are important habitats for brook trout; a species that along with fellow salmonid, the Atlantic salmon (Salmo salar), comprises a substantial component of the Maritime sport fishery. In Nova Scotia alone, over $53 million dollars of the provincial economy can be attributed to the angling of these fishes. The annual catch of brook trout and Atlantic salmon, however, is experiencing a relatively steady decline for a multitude of reasons; over-exploitation, habitat degradation, and the introduction of invasive species such as the smallmouth bass, for example, can account for some of this deterioration. In addition, brook trout are extremely sensitive to temperature-related stress and are intolerant to water temperatures above their thermal optima, which in general, is relatively low for salmonids. At present, climate change is but one factor that is contributing to increasing water temperatures on a global scale, including here in Atlantic Canada—also affecting water levels and flow rates. Many river and stream systems are warming to temperatures that are unsuitable for fish species like brook trout. As such, it is very important to develop and employ population estimate strategies that provide an accurate gauge of how these fish communities in lotic (or freshwater) ecosystems are faring in this time of expansive environmental change.
The term “census population size” refers to the total number of individuals in a population, in this case, the individuals being brook trout. This includes fish who have not yet reached sexual maturity or are otherwise incapable of producing offspring. The effective population size, on the other hand, is essentially an index of the genetic diversity in a population. In a paper published in 2016, Dr. Ruzzante and Mr. McCracken acknowledged “wide differences in population dynamics [of brook trout], probably resulting from differences in productivity affecting the intensity of competition for access to mates…” and suggested that “understanding the relationship between the effective population size, census population size, and the effective number of breeders, and how these relate to population dynamics and fluctuations in size, is important for the design of robust conservation strategies in small populations”.
A subsequent publication released in 2019 further addressed population estimation in brook trout; specifically, close-kin mark-recapture methods for estimating population abundance. In this study, abundance estimates from seven brook trout populations were compared using the standard mark-recapture (MR) and the close-kin mark-recapture (CKMR) methods, with the purpose of validating the latter as a method for estimating population size. This concept explores several aspects of population-related estimates; including their function as a necessity for the management and conservation of fishes native to both freshwater and marine environments.
The standard mark-recapture method involves the capture of a small number of individuals in a population, “marking” them in a way that theoretically causes them no harm and does not affect their behaviour, and subsequently releasing them. After a time-interval in which these marked individuals reintegrate into the population, another small subset of individuals is captured. The proportion of marked individuals in the second sample along with the total number of individuals marked can be used to infer population size. In small census populations, recapturing marked individuals is more probable, and the opposite is true for large census populations. Estimates using this method are most commonly applied in situations where it is impractical to count every individual. Close-kin mark-recapture, however, is rooted in the principle that an organism’s genetic material, or genotype, can be interpreted as a recapture of the genotype of each of its parents. If the paternal and maternal brook trout individuals are sampled independently of their progeny, the number of parent-offspring pairs identified by the presence of the same genetic markers in this study can then be used to estimate abundance. Adipose fin samples were digested to allow for the visualization of each fish’s genetic material, which was then used to determine intrapopulation relations.
“Despite various sources of error, [there was found to be] close agreement between standard MR abundance estimates…and CKMR estimates…[The] study constitutes the first in situ validation of CKMR and establishes it as a useful method for estimating population size in aquatic systems where assumptions of random sampling and thorough mixing of individuals can be met.”
Studies like those that Dr. Ruzzante, Mr. McCracken, and John MacMillan—along with their respective teams—are conducting are crucial in today’s environmental and political climate pertaining to the ecology, conservation, management, and regulation of species and their habitats. Brook trout—with their thermal sensitivity and long-running overexploitation—are a prime example of a species whose populations must be monitored as environmental parameters change drastically and the suitability of their ecosystems wane. As per the Nova Scotia Trout Management Plan—published by the Nova Scotia Department of Agriculture and Fisheries Inland Fisheries Division—assessing population size, migration pattern, and riparian habitats is a necessity to achieve a better understanding of population dynamics specific to brook trout. According to Dr. Ruzzante and Mr. McCracken, this study, while validating the CKMR approach for estimating population abundance when random sampling and thorough mixing of individuals can be assumed, can be applied to “harvested marine populations, populations of conservative concern, and those for which limited other information is available”. While it is unlikely that close-kin mark-recapture would be an effective method used to shed light on the population dynamics of systems comprising of tens of millions of fish or more, this approach is a gateway to accessing fundamental information pertaining to the conservation, management, and monitoring of a vast multitude of species found in Atlantic Canadian rivers and streams.
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