Accounting for spatial population structure at scales relevant to life history improves stock assessment: The case for Lake Erie walleye Sander vitreus

Accuracy and precision of fishery and demographic estimates from a spatial tag-recovery model when inter-regional movements are treated as fixed

“…Spatial structuring in the Lake Erie walleye population has been attributed to several factors, including metapopulation dynamics and regional differences in water quality, trophic structure, and incidence of invasive species (Wang et al, 2007;Berger et al, 2012). In addition to functioning as an apex predator (Ryan et al, 2003), walleye support important commercial and recreational fisheries throughout Lake Erie, with fishing activity mostly concentrated in the western and central basins (Roseman et al, 2011;Zhao et al, 2011).…”

“…Despite evidence to the contrary, existing SCAA models for west-central and eastern basin walleye stocks assume that the population is well mixed and that natural mortality does not vary by age or time Berger et al, 2012). Using a spatial tag-recovery model and recoveries from a long-term jawtagging study, Vandergoot and Brenden (2014) found that Lake Erie walleyes exhibited age and spatial variability with respect to movement and mortality components.…”

“…However, application of these models is infrequent and much still needs to be learned about their performance. Parameterization of spatial dynamics within SCAA assessment models is greatly facilitated by the integration of tagging data or other data sources that are informative about fish movement, although there are simpler approaches for accounting for spatial structure in SCAA models, such as the "fleets-as-areas" approach (Berger et al, 2012;Hurtado-Ferro et al, 2014). Even with these simpler approaches, it is likely that estimation of natural mortality rates using SCAA models will continue to be a challenge, particularly if regional estimates of natural mortality rates are desired.…”

Improving fishery-independent indices of abundance for a migratory walleye population

“…Instead there is consistent longitudinal spatial patterning of individuals by size (or age) such that a greater proportion of larger, older individuals tend to migrate further from spring spawning grounds in the west basin than smaller, younger individuals, apparently to optimize growth by taking advantage of seasonal water quality and foraging conditions favorable to these older fish (Berger et al, 2012;Kershner et al, 1999;Wang et al, 2007). Surveys occur in autumn during a time when migratory walleye are actively returning to the west basin (Wang et al, 2007;pers.…”

“…In this paper, we examine how site-level environmental variation in low dissolved oxygen (hypoxia), surface water temperature, water clarity and depth, and set type of survey gear influence walleye catch rates. The stock assessment model used to assess the status of Lake Erie walleye considers observed population trends from fishery-independent surveys as highly informative (i.e., surveys have comparatively more influence on how the model is fit than fishery-dependent data; Berger et al, 2012;WTG, 2010), and thus have considerable influence on resulting population estimates used for management. Therefore, it is critical to have a fishery-independent index that as best as possible accounts for factors that might confound real abundance trends.…”