Technical, Logistical, and Economic Considerations for the Development and Implementation of a Scottish Salmon Counter Network: Scottish Marine and Freshwater Science Vol 7 No 2

7.0 Spatial Considerations for a Counter Network

Background

Marine Scotland Science is planning on increasing the number of fish counters in Scottish rivers to improve the ongoing assessments of Atlantic salmon ( Salmo salar) abundance. In addition to site- and technology-specific requirements, and cost, it is important to consider the spatial distribution of a counter network when deciding on where to deploy additional counters. Specifically, the amount of spatial coverage should be considered. Spatial coverage, in the current context, can be defined as the percent of Scottish Atlantic salmon populations for which a counter-based estimate of abundance is available. Although at least one counter on every river is required for 100% coverage, the fact that salmonid populations covary (Cattaneo et al. 2003, Pyper et al. 2005, Kilduff et al. 2014) means that coverage of one river provides partial coverage of all correlated rivers. Mathematically, the coverage of the th river as provided by the countered rivers (c _x) could be defined as:

where,

and is the Pearson correlation coefficient between the th and th rivers. Total Scottish coverage (C, hereafter 'coverage index') is the mean of the riverine coverages weighted by the size of each population (N).

The coverage index relies on knowing correlations between streams. This could be problematic as counts are unavailable for most rivers. In the absence of such data, the correlations between rivers would have to be estimated from the rod catch data (Youngson 2002, Thorley et al. 2005). Similarly, the size of each population could be determined from the rod catches corrected for the inferred exploitation rate (Thorley et al. 2007).

Due to its flexibility, the coverage concept is a useful tool for thinking about counter deployment. When considering the addition of a single counter to an existing network, the coverage index can be used to compare competing sites. Alternatively, the concept can be used to compare alternative network arrangements. If the alternatives vary in price, then the criterion of choice can be the increase in coverage per unit expenditure. The weighting of rivers can also be adjusted to take into account additional considerations such as whether particular populations are of greater scientific or conservation concern. The concept also provides a framework for incorporating other sources of information such as traps and redd counts (Youngson et al. 2007) which can be scaled according to their accuracy relative to counters.

Challenges to the use of the coverage concept include the dependence on rod catches and multi-population rivers. Rod catches can vary regionally due to net catches, discharge, angler effort and other factors that are not directly related to abundance. This variation will tend to artificially inflate the correlation within rivers in the same region and deflate the correlation among rivers in different regions. A partial solution is to use the long-term trends to quantify the correlations (Youngson 2002). Larger Scottish rivers support multiple populations. The resultant spatio-temporal variation in abundance complicates the coverage concept but does not invalidate it. A workable solution is to redefine the concept so that small rivers and larger tributaries are the base units and to discount counters on the mainstems of large rivers with populations that cannot be separated by a combination of run-timing and fish size.

References

Cattaneo, F., Hugueny, B., and Lamouroux, N. 2003. Synchrony in brown trout, Salmo trutta, population dynamics: a 'Moran effect' on early-life stages. Oikos 100:43-54.
Kilduff, D.P., Botsford, L.W., and Teo, S.L.H. 2014. Spatial and temporal covariability in early ocean survival of Chinook salmon ( Oncorhynchus tshawytscha) along the west coast of North America. ICES Journal of Marine Science 71:1671-82.
Pyper, B.J., Mueter, F.J., and Peterman, R.M. 2005. Across-species comparisons of spatial scales of environmental effects on survival rates of northeast Pacific salmon. Transactions of the American Fisheries Society 134:86-104.
Thorley, J.L., Youngson, A.F., and Laughton, R. 2007. Seasonal variation in rod recapture rates indicates differential exploitation of Atlantic salmon, Salmo salar, stock components. Fisheries Management and Ecology 14: 191-98.
Thorley, J.L., Eatherley, D., Stephen, A., Simpson, I., Maclean, J., and Youngson, A. 2005. Congruence between automatic fish counter data and rod catches of Atlantic salmon ( Salmo salar) in Scottish Rivers. ICES Journal of Marine Science 62:809-17.
Youngson, A. 2002. Rod catch trends for early-running MSW salmon in Scottish Rivers (1952-1997): divergence among stock components. ICES Journal of Marine Science 59:836-49.
Youngson, A., MacLean, J.C., Bacon, P.J., Godfrey, J.D., Smith, G.W., and Thorley, J.L. 2007. Salmon Assessment in Scotland: Bringing information resources into line with recent research on methods. Scottish Fisheries Research Report No. 68. The Scottish Executive Environment Scottish Fisheries Research Report; Rural Affairs Department.