Otolith Shape Variability and Associated Body Growth Differences in Giant Grenadier, Albatrossia Pectoralis

Fish stocks can be defined by differences in their distribution, life history, and genetics. Managing fish based on stock structure is integral to successful management of a species because fishing may affect stocks disproportionately. Genetic and environmental differences can affect the shape and growth of otoliths and these differences may be indicative of stock structure. To investigate the potential for speciation or stock structure in giant grenadier, Albatrossia pectoralis, we quantified the shape of female giant grenadier otoliths and compared body growth rates for fish with three otolith shapes; shape types were classified visually by an experienced giant grenadier age reader, and were not defined by known distribution or life history differences. We found extreme variation in otolith shape among individuals; however, the shapes were a gradation and not clearly defined into three groups. The two more extreme shapes, visually defined as “hatchet” and “comb”, were discernable based on principal component analyses of elliptical Fourier descriptors, and the “mixed” shape overlapped both of the extreme shapes. Fish with hatchet-shaped otoliths grew faster than fish with comb-shaped otoliths. A genetic test (cytochrome c oxidase 1 used by the Fish Barcode of Life Initiative) showed almost no variability among samples, indicating that the samples were all from one species. The lack of young specimens makes it difficult to link otolith shape and growth difference to life history. In addition, shape could not be correlated with adult movement patterns because giant grenadiers experience 100% mortality after capture and, therefore, cannot be tagged and released. Despite these limitations, the link between body growth and otolith shape indicates measurable differences that deserve more study.

Fish stocks, identified for management purposes, are defined as being large enough to be self-sustaining and can be differentiated by their life histories [1]. An understanding of stock structure is integral to management when the productivity or population trends of stocks are not congruent [2]. Stocks can be defined by a variety of complementary methods including morphology, genetics, movement patterns, maturity, growth, and other life history characteristics (reviewed in [2]). Otolith shape morphology has been used extensively to aid in stock identification. Stock-specific shapes have been linked to disparate environmental conditions due to migrations to different feeding grounds or spawning areas [3–14], genetic differences [15], or differences in body condition and growth [3, 4, 6, 9, 13, 16]. In the literature, shape differences can be small enough that they cannot be detected without the aid of morphological analysis [3, 4, 12, 13, 14, 16, 17, 18].

Grenadiers (family Macrouridae) are deep-sea fish related to hakes and cods that occur globally in all oceans. Giant grenadier, Albatrossia pectoralis, have a wide geographic distribution, which extends from Baja California, Mexico around the arc of the North Pacific Ocean to Japan [19]. In Alaska, they are abundant in depths >400 m and are caught incidentally in other directed fisheries (total catch is estimated to be ~12,000 to 21,000 mt per year in Alaska) [19]. They have low commercial value, but are considered an important component of the ecosystem because of their large biomass (estimated biomass from 100–1,000 m in Alaska was 1.6 million mt in 2016) [20]. They may be susceptible to overfishing because they are late to mature (age at 50% maturity was estimated to be 23 years old [21]), ~95% of those caught in surveys are female, and all fish die after capture because of the pressure difference experienced when they are brought to the surface [19]. They are managed as two stocks in Alaska: eastern Bering Sea/Aleutian Islands and the Gulf of Alaska [19, 20]. This boundary is used for many federally managed species and is not specific to giant grenadier management.