Potential mechanisms, research avenues, and management action associated with size at age and growth of Pacific halibut

Over the last decade, the average size at age of Pacific halibut sampled in IPHC surveys and in commercial landings has declined markedly, and this trend has been cited as a concern with respect to various aspects of harvest policy. These declines have traditionally been attributed to declining growth rates. However, due to the nature of the observations that have been made, it remains unclear to what extent growth has actually declined as opposed to the changes in size at age being the result of other size-dependent processes. In particular, the average size at age in fish populations can be strongly influenced by size-dependent mortality. Mortality agents that preferentially remove small or slow-growing individuals (e.g., most predators) typically cause a cohort’s mean size at age to increase over time; agents that preferentially remove larger individuals (e.g., fishing mortality) can cause reductions in size at age. In this paper, a mechanistic framework is presented for distinguishing between growth, per se, and size at age in general, and a variety of natural and anthropogenic mechanisms is presented that can alter these features at either the individual or population level: size-selective predation; changes in food volume, quality, and assimilation rate; energy expenditure; intraand inter-specific resource competition; direct and indirect effects of fishing mortality; fisheries induced evolution. In each case, the conceptual basis of the potential mechanism is briefly presented, followed by a few research avenues that might be pursued in order to better understand its relative contribution to the population dynamics of Pacific halibut. Additionally, the potential for temporally-trended methodological and data biases to influence our perception of the magnitude and timing of such trends is discussed. This paper does not provide an exhaustive treatment of any single mechanism, nor does it present specific research designs or plans. Rather, its intent is to reflect the complexity of the process, highlighting the fact that growth need not change for size at age to be altered, and encourage a more thorough discussion of the topic than has yet occurred. It concludes with some thoughts on the conundrum of studying historical phenomena in the present, and prospects for management action in the absence of definitive answers.