Discrete seafloor features associated with juvenile haddock (Melanogrammus aeglefinus): A coarse-graining approach for identifying habitat scale

Abstract

Ecological patterns and processes occur at a myriad of spatial and temporal scales. Determining the appropriate scale(s) to collect and analyze data depends on the phenomena being studied and no one scale is appropriate for all of these. Knowing relevant measurement scales avoids the risk of presenting results that are merely a function of measurement scale rather than those relevant to actual ecological processes. Attribution of patterns to ecological processes requires a means to measure and evaluate these patterns. We present a standardized method to determine relevant spatial scales of fish-habitat associations observed from linear transect data on juvenile haddock (Melanogrammus aeglefinus), collected from six sites on the Scotian Shelf, off Canada's eastern coast. The survey data came from a large spatial scale research project undertaken by Fisheries and Oceans Canada to investigate spatial distribution of demersal fish in relation to seabed habitat in 2002 and 2003. We examined the relationship between two elements of this extensive dataset – the “towcam” video survey of demersal fish, and the interpreted surficial geology maps derived from sidescan sonar. We used the data from these surveys (48 towcam survey transect lines; 6 study sites, 2 5-km transect lines per site, day and night surveys per line, in each of 2 years = 240 km of transects), superimposed on the interpreted surficial geology layer collected at the same time. The resolution of the observations (∼2.5 m) facilitated a detailed examination of the spatial scales of juvenile haddock association with seabed habitat. This was achieved using a “coarse-graining” approach, which we applied by summarizing fish abundance and seabed habitat proportions across bin sizes starting at 5 m and progressively doubling to 640 m. We tallied significant correlations by bin size to observe patterns in fish-habitat associations as a function of measurement scale, for each age group and diel period. We found that Juvenile haddock-seabed associations deteriorated at the grain limits marking the range of spatial scaling relevant for this phenomenon. We present a robust approach for quantifying appropriate scales of aggregations across linear transects which can serve as an important tool for basing observations to describe ecological patterns in space.