# Comparison of Trophic Niche Position, Size, and Overlap in an Assemblage of Pacific Rockfish (Genus Sebastes) for Testing Community Composition Models

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

^{3}of tissue was sampled from the dorsal epaxial muscle (samples were comprised of muscle only). We froze all samples at −20 °C in the field and we transported them frozen to Brigham Young University where we stored them at −80 °C before we used them for stable isotope analysis. We prepared samples for analysis by oven-drying them at 60 °C for at least 48 h. Once desiccated, we ground samples to a homogeneous powder using a mortar and pestle then encapsulated them in a 4 × 6 mm tin capsule. Samples weighed between 0.6–1.2 mg. We shipped the samples to the Colorado Plateau Stable Isotope Laboratory at Northern Arizona University (Flagstaff, AZ, USA) for stable isotope analysis. Samples were analysed using a Delta V Advantage Mass Spectrometer (Thermo Electron Corporation, Bremen, Germany) and configured using a CONFLO III (Thermo Fisher Scientific, Waltham, MA, USA) and a Carlo Erba NC2100 Elemental Analyzer (Thermo Fisher Scientific, Waltham, MA, USA).

^{13}C) and nitrogen (δ

^{15}N) were expressed in permil (‰) using delta notation [30]. δ

^{13}C and δ

^{15}N were calculated using the equation:

^{13}C/

^{12}C or

^{15}N/

^{14}N and their associated international standard (carbon = Vienna Pee Dee Belemnite, nitrogen = atmospheric nitrogen; [31]).

^{13}C and δ

^{15}N ratios among pelagic and demersal rockfish to determine differences in trophic niche position among rockfish. We used two analysis of variance (ANOVA) tests for each sub assemblage to test for differences in δ

^{13}C and δ

^{15}N trophic niche axes. We structured these two ANOVA tests with species as the explanatory variable and δ

^{13}C or δ

^{15}N as dependent variables. We verified the assumptions of the parametric test by inspection of a residual plot to check for normality and constant variance. We compared means with 95% confidence intervals in a pairwise manner as a post hoc test to determine which species’ mean trophic niche positions differed within pelagic and demersal rockfish species. We determined that mean position differed if the mean δ

^{13}C or δ

^{15}N value did not overlap with the 95% confidence intervals of another species.

^{2}) for each rockfish species. We used the standard ellipse area to perform a pairwise comparison among rockfish species to quantify trophic niche overlap. We did this by calculating what percentage of the standard ellipse area of any given species was overlapping with any other rockfish species. All pairwise percent overlap estimates were rounded to the nearest whole percent. We took the sum of all these pairwise percent overlap estimates to be used as a metric that represents the total trophic niche overlap for each rockfish species. For example, a species with no trophic niche overlap would have a cumulative percent overlap of 0% while a complete niche overlap of four species on a fifth species would result in a cumulative percent overlap of 400%.

_{B}) using a Markov-chain Monte Carlo simulation at 10,000 iterations using normal priors [9]. Bayesian standard ellipses also contains the same properties of small sample size corrected standard ellipses for species with small samples sizes [9]. We compared these statistically by calculating the probability that the SEA

_{B}for a given rockfish was greater than the SEA

_{B}of another given rockfish. We used the equation:

_{B}of species i is greater than the SEA

_{B}of species j and the denominator is the total number of iterations ran in the Bayesian analysis. This returns a value we called p which is the probability that the Bayesian standard ellipse area of species i is greater than species j. We used these pairwise probabilities to determine if the trophic niche of a given rockfish was larger than that of any other given rockfish at a level of α = 0.05. We then provided a ranked list, with ties, of all species that characterized the position of each species’ trophic niche size from largest to smallest.

^{13}C or δ

^{15}N ratios differed in at least one of the years of our study. We used isotopic values from S. ciliatus, S. maliger, S. ruberrimus, and S. variabilis because we had large sample sizes across multiple years in the study (Table 1). We performed two ANOVA tests for each of these four species of rockfish. The dependent variable was either δ

^{13}C or δ

^{15}N measured in permil and the explanatory variable was year. We verified the assumptions of the parametric test by inspection of a residual plot to check for normality and constant variance.

^{13}C and δ

^{15}N by total length among seven species of rockfish. We used simple linear regression models for each rockfish species to determine if there was an ontogenetic shift in δ

^{13}C or δ

^{15}N. Species were determined to exhibit ontogenetic shifts if their regression had a non-zero slope. We compared slopes and 95% confidence intervals in a pairwise manner to determine which species’ ontogenetic changes in δ

^{13}C or δ

^{15}N differed from each other. We had sufficient samples sizes to test for ontogenetic relationships in seven rockfish species: S. brevispinis, S. ciliatus, S. flavidus, S. maliger, S. nigrocinctus, S. ruberrimus, and S. variabilis (Table 1). We verified the assumptions of the parametric test by inspection of a residual plot to check for normality and constant variance.

## 3. Results

#### 3.1. Trophic Niche Position

^{13}C ranged from −17.96 to −16.70 and for δ

^{15}N from 12.95 to 14.21 (Figure 1). There was a significant difference in mean trophic niche position in both the carbon (F

_{4, 357}= 29.07, p < 0.001) and nitrogen (F

_{4, 357}= 56.97, p < 0.001) niche axes among pelagic rockfish species. Sebastes ciliatus and S. variabilis were more depleted in both mean carbon and mean nitrogen than all other rockfish (Figure 1). Sebastes melanops was not significantly different from S. flavidus or S. brevispinis in mean carbon or nitrogen position, but S. flavidus was significantly more depleted than S. brevispinis in mean nitrogen position (Figure 1).

^{13}C varied from −16.03 to −15.78 and for δ

^{15}N from 14.26 to 15.58 (Figure 1). There was no significant difference in the mean carbon trophic niche position (F

_{4, 345}= 1.9853, p = 0.096) but there was a significant difference in mean nitrogen position among demersal rockfish species (F

_{4, 345}= 53.64, p < 0.001). Sebastes ruberrimus and S. nigrocinctus were significantly more enriched than all other demersal rockfish in mean nitrogen position (Figure 1). Sebastes maliger was more enriched in mean nitrogen position than S. caurinus, but there was no difference between S. maliger and S. nebulosus (Figure 1).

#### 3.2. Trophic Niche Size

^{2}(S. caurinus) to 1.313‰

^{2}(S. brevispinis; Table 2, Figure 3). Bayesian standard ellipses showed that S. brevispinis had the highest estimated SEA and SEA

_{B}but was tied with S. melanops and S. variabilis (Table 2, Figure 3). Sebastes nigrocinctus had the smallest niche size among all rockfish (Table 2, Figure 3). In general, pelagic rockfish had larger SEA and SEA

_{B}than demersal rockfish (Table 2, Figure 3).

^{2}for pelagic rockfish and 0.958‰

^{2}for demersal rockfish. Mean trophic niche size predicted by the null model was 3.5‰

^{2}for pelagic rockfish and 2.5‰

^{2}for demersal rockfish (Figure 4).

#### 3.3. Trophic Niche Overlap

#### 3.4. Interannual Differences

^{13}C values across years (Table 4). There was not a significant difference among years for S. ciliatus or S. variabilis in δ

^{13}C (Table 4, Figure 6). Sebastes ruberrimus, S. ciliatus, and S. variabilis were significantly different in the δ

^{15}N values across years (Table 5). Sebastes maliger was not significantly different in δ

^{15}N across years (Table 5, Figure 7).

#### 3.5. Ontogenetic Differences

^{2}= 0.1488, F

_{1, 148}= 27.05, p = 6.5 × 10

^{−7}) and S. variabilis (R

^{2}= 0.04698, F

_{1,130}= 7.458, p = 0.00126) exhibited increased enrichment of δ

^{13}C with increasing total length. None of the other species exhibited a significant relationship between δ

^{13}C and total length (Figure 8).

^{15}N relationship with total length. Sebastes brevispinis (R

^{2}= 0.5252, F

_{1,27}= 31.97, p = 0.00138), S. ciliatus (R

^{2}= 0.2127, F

_{1, 148}= 41.27, p = 0.00068), S. maliger (R

^{2}= 0.1384, F

_{1, 155}= 26.05, p = 9.6 × 10

^{−7}), S. nigrocinctus (R

^{2}= 0.2002, F

_{1, 44}= 12.26, p = 0.00132), and S. variabilis (R

^{2}= 0.3502, F

_{1, 130}= 71.61, p = 0.00107) did not differ significantly by slope (Figure 9). Sebastes brevispinis, S. ciliatus, and S. variabilis had a slope of δ

^{15}N with length that was more than twice as large as the slope for S. ruberrimus (R

^{2}= 0.2982, F

_{1, 124}= 54.12, p = 2.26 × 10

^{−11}), but S. ruberrimus did not differ significantly from S. maliger and S. nigrocinctus in slope (Figure 9).

## 4. Discussion

^{15}N axis and in both the δ

^{15}N and δ

^{13}C axes for pelagic rockfish (Figure 1). The mean position was different than the null model for both δ

^{13}C and δ

^{15}N (Figure 2) in both sub-assemblages we sampled. There were differences in niche size among rockfish species (Table 2), but overall niche sizes were substantially smaller than those predicted by the null model for both pelagic and demersal rockfish groups (Figure 4). Finally, there was relatively little niche overlap among rockfish species and no niche overlaps exceeded a two-fold overlap with all other rockfish species (Table 3). The null model predicted that trophic niche overlap should on average be about 350%, and always exceed about 250%. Thus, at this scale of investigation (i.e., among ten species), all niche characteristics we measured suggest that this assemblage is organized by resource partitioning processes resulting in niche packing characteristic of the deterministic model of community composition. Given the relatively large number of similar coexisting species in this assemblage, and the small total amount of niche space occupied by this assemblage (i.e., about one trophic level on the δ

^{15}N axis, 3‰, and about 3‰ on the δ

^{13}C axis), we found it surprising that each species occupied a relatively distinct part of trophic niche space.

^{13}C varied by less than 0.5‰ and δ

^{15}N varied by less than 1‰ among years, and similarly, the largest ontogenetic change in δ

^{15}N was by S. brevispinis which changed by approximately 1.5‰. This ontogenetic change in S. brevispinis probably contributed to the relatively large niche size observed for this species, but we note that resulting cumulative overlap for this species was still relatively low at 121% compared to that expected from a lottery-type model. Additionally, the size range of the fish that are sampled directly affects the ontogenetic shift that is observed. We sampled 80% of the size range of S. brevispinis, 55% of the size range of S. maliger, and 87% of the size range of S. ruberrimus [14,40]. We sampled 50% of the entire size range for S. ciliatus, S. flavidus, and S. variabilis. Size at maturity has not been published for S. nigrocinctus. We suggest that when using niche characteristics to differentiate between different models of community composition and species coexistence, it is important to quantify potential sources of variation in the measured niche metrics that might inflate estimates.

^{13}C stable isotope ratios of the consumer reflect that of their forage and are not enriched trophically [41,43,44]; whereas δ

^{15}N stable isotope ratios enrich at predictable increments in the food chain [31,41,43,45]. Because of these properties, similarities in stable isotope ratios of consumers can appear to come from the same source if multiple sources exhibit similar isotopic values. Thus, similarities in stable isotope ratios may not actually represent real similarities in diet if two species are using different prey resources that result in similar isotopic compositions. If this occurs, we could assume equivalence in trophic niches when in fact diets are taxonomically different. However, our results are somewhat conservative relative to this point because we found isotopic differences in trophic niche position among most species. In the few cases noted above where two or three species have similar niches, it is possible that these similarities may result from taxonomically different, but isotopically similar prey. If this is the case, our support for the deterministic species coexistence model is strengthened. In future studies, it would be important to quantify and determine isotopic signatures of prey species along with the target assemblage.

^{13}C axis in addition to the δ

^{15}N axis, there may be additional forage items that allow them to have a larger niche breadth. While there has been evidence of this in South American amphibians, it appears that rockfish exhibit a pattern that would warrant additional studies to see if rockfish niche breadth is affected by prey diversity.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Mean isotopic δ values and 95% confidence intervals for ten rockfish species from southeast Alaska. Species names in bold font are demersal and non-bold names are pelagic. The five species of pelagic rockfish are less than −16.5‰ δ

^{13}C and the five species of demersal rockfish are greater than −16.5‰ δ

^{13}C.

**Figure 2.**Comparison of observed trophic niche position and the null model distribution for pelagic and demersal rockfish. Each panel shows either the δ

^{13}C or δ

^{15}N null distributions based on the resampling of the isotopic values for pelagic (

**A**,

**B**) or demersal rockfish (

**C**,

**D**). Observed means for each rockfish species are represented by the bolded vertical lines.

**Figure 3.**Standard ellipses for the rockfish assemblage. Species names in bold font are demersal and pelagic species are in non-bold font.

**Figure 4.**Comparison of trophic niche size for pelagic and demersal rockfish. Each panel shows the SEA distributions based on the resampling of the isotopic values for pelagic (

**A**) or demersal rockfish (

**B**). Actual SEA of each rockfish species is represented by the bolded vertical lines.

**Figure 5.**Comparison of cumulative trophic niche overlap for pelagic and demersal rockfish. Distributions of the cumulative percent trophic niche overlap based on the resampling of the isotopic values (i.e., null model) for pelagic (

**A**) or demersal rockfish (

**B**). Observed cumulative percent trophic niche overlap of each rockfish species is represented by the bolded vertical lines.

**Figure 6.**Mean shifts in δ

^{13}C and 95% confidence intervals across time for four rockfish species in this assemblage.

**Figure 7.**Mean shifts in δ

^{15}N and 95% confidence intervals across time for four rockfish species in this assemblage.

**Figure 8.**Ontogenetic shifts in δ

^{13}C by total length for seven rockfish species in this assemblage.

**Figure 9.**Ontogenetic shifts in δ

^{15}N by total length for seven rockfish species in this assemblage.

Species | 2013 | 2014 | 2015 | 2016 | 2017 | 2018 | Total | Size (TL) |
---|---|---|---|---|---|---|---|---|

S. brevispinis | 0 | 2 | 15 | 5 | 4 | 3 | 29 | 324–568 mm |

S. ciliatus | 0 | 0 | 0 | 52 | 62 | 36 | 150 | 288–554 mm |

S. flavidus | 1 | 15 | 20 | 0 | 2 | 0 | 38 | 330–640 mm |

S. melanops | 0 | 0 | 1 | 8 | 1 | 3 | 13 | 260–597 mm |

S. variabilis | 0 | 0 | 0 | 40 | 61 | 31 | 132 | 287–513 mm |

S. caurinus | 4 | 2 | 0 | 0 | 1 | 0 | 7 | 350–390 mm |

S. maliger | 22 | 36 | 15 | 31 | 28 | 25 | 157 | 295–660 mm |

S. nebulosus | 3 | 0 | 2 | 8 | 0 | 1 | 14 | 340–420 mm |

S. nigrocinctus | 6 | 4 | 20 | 5 | 6 | 5 | 46 | 350–616 mm |

S. ruberrimus | 18 | 12 | 15 | 24 | 31 | 26 | 126 | 252–857 mm |

**Table 2.**Standard ellipse area (SEA) and Bayesian standard ellipse area (SEA

_{B}) expressed in ‰

^{2}and ranked by size, including ties, for ten rockfish species from southeast Alaska, USA.

Species | SEA | SEA_{B} | Rank |
---|---|---|---|

S. brevispinis | 1.3130 | 1.3711 | a |

S. variabilis | 1.0290 | 1.0180 | abc |

S. melanops | 0.8205 | 1.1041 | abc |

S. ciliatus | 0.9716 | 0.9717 | bc |

S. ruberrimus | 0.9580 | 0.9675 | bc |

S. flavidus | 0.7295 | 0.7578 | bcd |

S. maliger | 0.7347 | 0.7465 | bcd |

S. nebulosus | 0.4059 | 0.5727 | d |

S. caurinus | 0.3643 | 0.4905 | d |

S. nigrocinctus | 0.3882 | 0.4414 | e |

**Table 3.**Pairwise and cumulative niche overlaps among all rockfish in the assemblage. Bolded terms are the cumulative percent overlap from all rockfish species and is the sum of all pairwise overlaps from each column.

S. brevispinis | S. ciliatus | S. flavidus | S. melanops | S. variabilis | S. caurinus | S. maliger | S. nebulosus | S. nigrocinctus | S. ruberrimus | |
---|---|---|---|---|---|---|---|---|---|---|

S. brevispinis | 121 | 10 | 79 | 77 | 0 | 9 | 15 | 27 | 0 | 0 |

S. ciliatus | 7 | 105 | 33 | 19 | 51 | 0 | 0 | 0 | 0 | 0 |

S. flavidus | 43 | 25 | 185 | 58 | 1 | 0 | 0 | 3 | 0 | 0 |

S. melanops | 51 | 17 | 69 | 196 | 1 | 15 | 24 | 22 | 3 | 1 |

S. variabilis | 0 | 53 | 2 | 1 | 54 | 0 | 0 | 0 | 0 | 0 |

S. caurinus | 3 | 0 | 0 | 7 | 0 | 109 | 33 | 29 | 0 | 0 |

S. maliger | 8 | 0 | 0 | 20 | 0 | 56 | 156 | 80 | 51 | 6 |

S. nebulosus | 9 | 0 | 2 | 11 | 0 | 29 | 47 | 189 | 28 | 0 |

S. nigrocinctus | 0 | 0 | 0 | 2 | 0 | 0 | 30 | 28 | 136 | 25 |

S. ruberrimus | 0 | 0 | 0 | 1 | 0 | 0 | 7 | 0 | 54 | 32 |

Coefficients | Sum of Squares | Degrees of Freedom | F-Value | p-Value |
---|---|---|---|---|

S. ruberrimus | 4.757 | 1 | 14.68 | 2.01 × 10^{−4} |

S. maliger | 0.9861 | 1 | 4.891 | 0.02846 |

S. ciliatus | 0.1563 | 1 | 0.2984 | 0.5857 |

S. variabilis | 1.556 | 1 | 3.038 | 0.08371 |

Coefficients | Sum of Squares | Degrees of Freedom | f-Value | p-Value |
---|---|---|---|---|

S. ruberrimus | 1.999 | 1 | 7.765 | 0.006164 |

S. maliger | 0.6451 | 1 | 2.172 | 0.1426 |

S. ciliatus | 0.8088 | 1 | 3.683 | 0.05689 |

S. variabilis | 7.028 | 1 | 39.73 | 4.183 |

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## Share and Cite

**MDPI and ACS Style**

Suchomel, A.D.; Belk, M.C.
Comparison of Trophic Niche Position, Size, and Overlap in an Assemblage of Pacific Rockfish (Genus *Sebastes*) for Testing Community Composition Models. *Diversity* **2022**, *14*, 689.
https://doi.org/10.3390/d14080689

**AMA Style**

Suchomel AD, Belk MC.
Comparison of Trophic Niche Position, Size, and Overlap in an Assemblage of Pacific Rockfish (Genus *Sebastes*) for Testing Community Composition Models. *Diversity*. 2022; 14(8):689.
https://doi.org/10.3390/d14080689

**Chicago/Turabian Style**

Suchomel, Andrew D., and Mark C. Belk.
2022. "Comparison of Trophic Niche Position, Size, and Overlap in an Assemblage of Pacific Rockfish (Genus *Sebastes*) for Testing Community Composition Models" *Diversity* 14, no. 8: 689.
https://doi.org/10.3390/d14080689