When Northwest Atlantic predatory groundfish experienced a collapse in numbers in the early 1990s, one result was the creation of a 'natural experiment' which could be used to explore responses of ecosystems to a major disturbance. The Pelagic Outburst hypothesis was developed to explain an increase of up to 900% in the abundance of small-bodied forage fishes and macroinvertebrates following this collapse and a subsequent cascade across four trophic levels.
Recently though this theory has been challenged, and an alternative hypothesis, Suprabenthic Habitat Occupation (SHO), has been advanced. SHO proposes that the prey outburst associated with the forage fish component was an illusion created by post-cod collapse changes in the vertical distribution of small pelagic fishes that saw them become distributed towards the bottom of the ocean, increasing their vulnerability to bottom trawls.
The authors evaluated the SHO hypothesis as it applied to the relationship between changes in the biomass of cod and the vertical distribution of herring and sand lance, the two major small pelagic species of the Scotian Shelf ecosystem off eastern Nova Scotia.
Contrary to predictions, the authors initial conclusion that a pelagic outburst occurred in that ecosystem was confirmed, and no evidence was found of a predator effect on vertical distributions of these species. They also explored the acoustic survey design and execution that lead to the data behind the SHO hypothesis as well as the coherence between the behaviour depicted in these data and catch rates in the surface-oriented purse seine herring fishery operating at the time of the surveys.
In combination, the results of the reanalysis of the population dynamics and behaviour of eastern Scotian Shelf herring, lead the authors to deduce that SHO, at least in terms of the post cod-collapse dynamics in northwest Atlantic ecosystems, is not supported.