DOI: 10.5061/dryad.mw6m90651
title: Variation in isotopic trophic niche of sablefish (Anoplopoma fimbria) and shortraker rockfish (Sebastes borealis) in the Northeast Pacific

description: 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.
description: Sampling Areas We obtained samples of
 <em>A. fimbria</em> and <em>S. borealis</em> 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 <em>A.
 fimbria</em> and 26 <em>S. borealis</em>) 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 <em>A.
 fimbria</em> and <em>S. borealis</em> for stable isotope
 analysis according to standard methods. Briefly, we dried approximately 1
 cm<sup>3</sup> 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 δ<sup>15</sup>N
 and δ<sup>13</sup>C 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 (δ<sup>15</sup>N) and carbon
 (δ<sup>13</sup>C) isotopes in permil (‰) using the δ notation.
 Both δ<sup>13</sup>C and δ<sup>15</sup>N values
 were calculated using the equation: δ<sup>H
 </sup>X = [( ) − 1] · 1000 where the δ
 notation is specified for either X = N or X = C, the <sup>H
 </sup>denotes the heavy isotope mass, and the <em>R</em>
 represents the ratio of
 <sup>15</sup>N/<sup>14</sup>N or
 <sup>13</sup>C/<sup>12</sup>C [18].
 To account for variable muscle lipid content in <em>A.
 fimbria</em> and <em>S. borealis </em>and thus potential
 bias in observed δ<sup>13</sup>C, we adjusted
 δ<sup>13</sup>C values using a normalized lipid correction
 factor based on C:N ratios. We adjusted the δ<sup>13</sup>C
 isotope ratios of <em>A. fimbria</em> and <em>S.
 borealis</em> 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 &lt; 4 (i.e., low lipid content) do
 not require adjustment. C:N ratios for <em>A. fimbria</em>
 were all above this minimum and C:N ratios for <em>S.
 borealis</em> were almost all below this minimum; however, we
 adjusted δ<sup>13</sup>C values for all samples of both
 species to be consistent.
description: # Variation in isotopic trophic niche of sablefish (*Anoplopoma fimbria*)
 and shortraker rockfish (*Sebastes borealis*) in the Northeast Pacific
 [https://doi.org/10.5061/dryad.mw6m90651](https://doi.org/10.5061/dryad.mw6m90651) ## Description of the data and file structure ### Files and variables #### File: Sablefish\_and\_Shortraker\_Alaska\_Data\_updated\_for\_Dryad\_aug\_2024.xlsx **Description:** excel sheet containing all data used for the paper ##### Variables | Sample ID | Date | Species | Location | Total Length (mm) | d15N vs. At. Air (‰) | d13C vs. VPDB (‰) | d13C Corrected | | :----------------------------------- | :------------------------------------------ | :------ | :---------------------------------------------------------------------------------------------------------------------- | :----------------------------------------- | :------------------------------------------------------------------------ | :--------------------------------------------------------------------------------- | :------------------------------------------ | | A unique identifier of each sample.  | The date that each sample was collected on. | species | Inside = a sampling area in Lynn Canal which is just north of the eastern extent of Icy Strait in southeast Alaska, USA | Total length was expressed in millimeters. | δ15N isotope values were expressed in permil relative to atmospheric air. | δ13C isotope values were expressed in permil relative to Vienna PeeDee Belemnite.  | δ13C isotope values after lipid correction. | | | | | Coastal = a sampling area near the open ocean by the western outlet of Icy Strait in southeast Alaska, USA. | | | | |