Sablefish (Anoplopoma fimbria, Anoplopomatidae) and shortraker rockfish (Sebastes borealis, Sebastidae), co-occur in deepwater marine habitats in the northeast Pacific. Both species are economically valuable, but their ecologies are not well known. We used stable isotope analysis of carbon and nitrogen to explore isotopic niches of A. fimbria and S. borealis in two distinct locations – a deep strait in the inside passage area and an open coastal area of the continental shelf, both in southeast Alaska, USA. Anoplopoma fimbria and S. borealis exhibited similar positions of isotopic niches based on nitrogen and carbon isotopic ratios suggesting potential interspecific competition, especially in the inside location. In addition, S. borealis had a smaller niche breadth compared to A. fimbria in the coastal location. Both species had enriched nitrogen and carbon isotopic ratios in the inside location compared to the coastal location. Differences in isotopic niches between these two locations suggest the possibility of location-specific variation in isotopic niches of these two species of widespread, abundant deepwater fishes.

Sampling Areas

We obtained samples of A. fimbria and S. borealis in two distinct locations in southeast Alaska (Figure 1) from June to August 2021. Both locations exhibited an average annual air temperature of 4.83 °C and water depth and thus sampling depth was 600 to 700 meters at both locations. The first sampling area was out on the open ocean near the western outlet of Icy Strait which will be referred to as the coastal location. The second area was in Lynn Canal, a narrow, deep strait located just north of the eastern extent of Icy Strait. This location will be referred to as the inside location (Figure 1). Although located at the same latitude and within 100 km of each other, these two locations represent the most extreme differences in habitat where both species occur as adults.

Samples of both species (62 A. fimbria and 26 S. borealis) were obtained from recreational fishermen in the inside location and commercial catches in the coastal location (Figure 1). All fish were caught near the bottom substrate and these species are not known to inhabit the water column. We measured the standard length (SL) and total length (TL) of each fish in millimeters (Table 1). Muscle tissue samples were taken from the epaxial muscle just behind the head (about 4 cm long and 1 cm wide). Samples were kept frozen until processed for analysis. We compared the lengths of each species in both locations using a two-sample t-test (α = 0.05).

We prepared muscle tissue samples of A. fimbria and S. borealis for stable isotope analysis according to standard methods. Briefly, we dried approximately 1 cm³ of muscle tissue in a 60 °C dehydrator for 48 hours. Following dehydration, we ground the samples using a m­ortar and pestle until they were homogenized into a fine powder. We then measured 0.7 to 1.2 mg of powder for each sample and transferred it into 3 x 5 mm tin capsules. The samples were analyzed for δ¹⁵N and δ¹³C stable isotopes at the Cornell University Stable Isotope Laboratory in Ithaca, New York. Analysis was performed using a Thermo Delta V isotope mass spectrometer coupled with an NC2500 analyzer. Stable isotope values of samples were compared to a set standard of PeeDee Belemnite (PDB) for carbon and atmospheric nitrogen (AIR) for nitrogen. We expressed nitrogen (δ¹⁵N) and carbon (δ¹³C) isotopes in permil (‰) using the δ notation. Both δ¹³C and δ¹⁵N values were calculated using the equation:

δ^H X = [( ) − 1] · 1000

where the δ notation is specified for either X = N or X = C, the ^H denotes the heavy isotope mass, and the R represents the ratio of ¹⁵N/¹⁴N or ¹³C/¹²C [18].

To account for variable muscle lipid content in A. fimbria and S. borealis and thus potential bias in observed δ¹³C, we adjusted δ¹³C values using a normalized lipid correction factor based on C:N ratios. We adjusted the δ¹³C isotope ratios of A. fimbria and S. borealis mathematically by using the following equation:

We determined the C:N ratio by dividing the percent carbon (from the stable isotope analysis) by the percent nitrogen (from stable isotope analysis) and then multiplying by 14/12 (the atomic mass of nitrogen over the atomic mass of carbon) to get a molar estimate of C:N. Generally, C:N ratios of < 4 (i.e., low lipid content) do not require adjustment. C:N ratios for A. fimbria were all above this minimum and C:N ratios for S. borealis were almost all below this minimum; however, we adjusted δ¹³C values for all samples of both species to be consistent.