Jonathan J. Deroba

Evaluating the Consequences of Adjusting Fish Stock Assessment Estimates of Biomass for Retrospective Patterns using Mohn's Rho

AbstractA retrospective pattern exists when estimates of some assessment parameter for a given time period (e.g., biomass in a given year) trend in a systematic way as additional periods of data (i.e., typically a year) are added. Causes of such patterns are difficult to determine, and the pattern does not necessarily indicate the direction of the parameter bias. Mohns rho measures the severity of retrospective patterns and has been used to adjust estimates of biomass and quotas. The consequences of applying Mohns rho rather than ignoring the pattern are unclear. I used an age-structured simulation based on Atlantic Herring Clupea harengus to evaluate the consequences of ignoring retrospective patterns versus applying Mohns rho. When the application of Mohns rho produced more accurate estimates of biomass, subsequent spawning stock biomass and yields were near target levels. When the use of Mohns rho produced less-accurate estimates of biomass, the results depended on the direction of the pattern. Wi...

Sources of variation in stomach contents of predators of Atlantic herring in the Northwest Atlantic during 1973–2014

Spatial and temporal variation in stomach-contents data is often unquantified or combined in such a way (e.g. averaged among years) that true signal in diets may be lost. Using a delta approach, this paper fits generalized additive mixed models (GAMMs) to the amount of Atlantic herring (Clupea harengus) identified in predator stomachs using only data from stomachs in which herring occurred, and to the probability that a stomach contained herring. Both the amount of herring in stomachs and the probability of a stomach containing herring varied seasonally, spatially, and among years. Of the random effects in each GAMM, the effect of predator species had the largest variance. An index of herring abundance derived from the stomach-contents data was generally consistent with recent herring stock assessments. The temporal and spatial variation in the stomach-contents data suggested that the effect of averaging or combining stomach-contents data among years, seasons, or areas may lead to falsely precise or biased estimates from multispecies assessments or in estimates of consumption, and may restrain the relevance of static foodweb models.