Environmental and Ecological Drivers of Slow Growth in Deep-Sea Demersal Fishes

Abstract

ABSTRACT: The deep sea (>500m ocean depth) is the largest habitat on the planet, characterized by near-freezing temperatures, low ambient light, and food-poor conditions relative to shallower waters. Fishes in the deep sea generally grow more slowly than those inhabiting shallow-water. This is a generalization, however, and even amongst deep-sea fishes, there is a broad continuum of growth rates. The relative importance of potential drivers of growth-rate variability amongst deep-sea species, such as temperature, food availability, oxygen concentration, metabolic rate, and phylogeny, have yet to be fully evaluated. We present a meta-analysis in which age and size data were collected for 53 species of fishes whose collective depth ranges span surface waters to 4000m. Here we focus on demersal species because much of the existing data for deep-sea fish growth is centered on commercially harvested, demersal taxa. We calculated growth metrics using both calendar and thermal age, and compared them with environmental, ecological, and phylogenetic variables. Temperature alone explains up to 30% of variation in the Von Bertalanffy growth coefficient, K (yr-1), and 21% of the variation in the average annual increase in mass (AIM; %), a metric of growth prior to maturity. After correcting for the influence of temperature, depth was still a significant driver of growth, explaining up to 20% and 10% of the remaining variation in K and AIM, respectively. Oxygen concentration also explained ~11% of remaining variation in AIM following temperature-correction. Relatively minor amounts of variation may be explained by food availability and the locomotory mode of the fishes. We also found a strong correlation between growth and metabolic rate. Deeper, slower-growing stocks are generally more vulnerable to overfishing due their relatively slow growth rates, though considerable variation in growth persists, even amongst deep-sea fishes. By understanding the influence of multiple ecological and/or environmental drivers of growth rate, we can better estimate resilience to fishing mortality than from depth alone. Further disruption of fish populations and habitats may be compounded by the advent of deep-sea mining for rare minerals. Deep-sea mining may begin within the next few decades and will target a diversity of deep-sea habitats across the globe, including seamounts, mid ocean ridges, and abyssal plains. It is therefore vital to accurately model the growth rates of deeper-living fish to evaluate their resiliency to escalating anthropogenic disturbance.