Biology, Assessment, and Management of North Pacific Rockfishes 21 Sea Grant College Program • AK-SG-07-01, 2006

Rockfish Trophic Relationships in Prince William Sound, Alaska, Based on Natural Abundance of Stable Isotopes Thomas C. Kline Jr. Prince William Sound Science Center, Cordova, Alaska

Abstract About two dozen rockfish species coexist within Prince William Sound (PWS). According to ecological theory these species should reduce competition by minimizing diet overlap. This was verified by using eco- logical metrics based on the natural abundance of carbon and nitrogen stable isotopes. Carbon source dependency was based on a regional carbon isotope gradient whereas trophic level was based on trophic enrichment of 15N. There was a gradient in carbon source dependency among rockfish slope, pelagic, and demersal eco-groups. Pelagic rock- had the greatest dependency on Gulf of Alaska (GOA) derived car- bon. Within demersal rockfishes, were most dependent on PWS derived carbon. Rockfishes did not respond to a strong pulse in GOA subsidies, like forage groups, confirming that their stable isotope composition reflected relatively longer time-integration. Yelloweye and shifted concordantly in carbon source dependency but separated in terms of relative trophic level. Relative trophic level (TL), based on the nitrogen isotope value of a herbivorous for reference TL = 2.0, ranged from 3.2 (juveniles) to 5.1 (shortspine thorny- head). Another slope species, the shortraker rockfish, had the second highest TL = 4.9. Dark rockfish was the lowest adult TL = 3.6. Alternative nonlethal sampling for isotope values using blood is possible given normalization of the data for lipid isotope effects.

Introduction In the northeast Pacific Ocean, the scorpaenid fishes of the genera and , commonly called rockfishes, comprise more 22 Kline—Rockfish Trophic Relationships

Table 1. Twenty-five rockfishes found in PWS from the literature and this study.

Meyer Rosenthal This Species Common name 1992 1980 study

Sebastes melaonostomus Blackgill rockfish × Sebastes melanops × × × Sebastes paucispinis Bocaccio × Sebastes auriculatus × × Sebastes pinniger Canary rockfish × Sebastes nebulosus × × × Sebastes caurinus Copper rockfish × × × Sebastes ciliatus Dark rockfisha × × × Sebastes elongatus Greenstripe rockfish × Sebastes variegatus Harlequin rockfish × Sebastes alutus × Sebastes emphaeus Puget Sound rockfish × Sebastes maliger Quillback rockfish × × × Sebastes babcocki Redbanded rockfish × Sebastes proriger Redstripe rockfish × × Sebastes helvomaculatus Rosethorn rockfish × Sebastes aleutianus Rougheye rockfish × × Sebastes zacentrus Sharpchin rockfish × Sebastes borealis Shortraker rockfish × × Sebastes brevispinis Silvergray rockfish × × × Sebastes diploproa Splitnose rockfish × Sebastes nigrocinctus Tiger rockfish × × × Sebastes ruberrimus × × × Sebastes flavidus Yellowtail rockfish × × × Sebastolobus alascanus Shortspine thornyhead × ×

aFor nomenclature see Orr and Blackburn 2004.

than 60 species. Twenty-five of these species have been found in Prince William Sound (PWS), Alaska, a subarctic marine ecosystem of about 100 by 100 km (Table 1). Because much of PWS is north of 60ºN, this is the northern limit for some rockfish species, while others are also found in the higher latitude portions of the Bering Sea. PWS has depths to about 800 m thus providing a good range of depths accommodating the varying depth range preferences among the many rockfish species (Love et al. 2002). Ecological theory, which suggests rockfish species coexisting within PWS reduce would compete by minimizing diet overlap, was tested Biology, Assessment, and Management of North Pacific Rockfishes 23 for adults of twelve rockfish Sebastes( and Sebastolobus) species and juvenile Sebastes using stable isotope analysis (SIA) as a metric of diet overlap (Welch and Parsons 1993, Murie 1995). Application of SIA methods was enhanced by an existing isotopic context of potential prey sampled at the same time as most of the rock- fishes (Fig. 1). A regional stable carbon isotope gradient can be used to differentiate the relative dependency of autochthonous production sources (from PWS) versus allochthonous sources assumed to be from the Gulf of Alaska (GOA; Kline 1999). Trophic level (TL) can be estimated based on nitrogen stable isotope values assuming a consistent nitrogen isotope trophic enrichment (Kline and Pauly 1998; Kline 2001, 2002; Wada et al. 1991). The mean δ13C′ value (see Materials and methods for definition) of whole individual Neocalanus analyzed from GOA was –22.7 (SD = 2.6, SE <0.1, N = 1,590) during 1995-2004; whereas that from PWS was –19.8 (SD = 2.0, SE <0.1, N = 758) (Kline 1999, 2001, 2002, and unpubl. data for post-1996). Carbon δ13C′ values less than about –20.5 were hypothesized to reflect allochthonous carbon with respect to PWS, reflecting GOA production sources. In contrast, theδ 15N gradient is much weaker (Kline 1999). The decadal mean δ15N for PWS was 8.9 (SD = 1.9, SE <0.1, N = 752); whereas GOA was 7.3 (SD = 2.5, SE <0.1, N = 1,588). Because of the gradient, though small, when a δ15N value of 8.4 is used as the reference for calculating TL, the uncertainty is about 0.3 TL (Kline 2001). Nevertheless, there was good agreement between TL based on δ15N and TL based on predictions made by the Ecopath model (Kline and Pauly 1998). Furthermore, TL of juvenile herring, which is possibly the most important forage in PWS, based onδ 15N was consistent, about 3.2 ± <0.2 over a four-year period (Fig. 1). A goal of this study was to determine whether PWS rockfishes parti- tion food resources in terms of carbon source and relative trophic level with respect to species as well as the three depth range eco-groups: de- mersal (shelf species associated with substrates), pelagic (shelf species associated with the upper water column), and slope (deep-dwelling spe- cies, generally found deeper than 200 m). Accordingly, synoptic mean 13 15 δ C′TL and mean δ N values of eco-groups or species should differ. Rockfishes are long-lived with relatively slow growth. The stable isotope values of adults were thus expected to reflect or integrate longer time spans than fishes with short life spans (cf. the 2 to 6 years of Pacific salmon species). Accordingly, their isotope values were not expected to vary much over times of less than ~1 year (Hesslein et al. 1993). For example, the large shifts in δ13C′ value were not expected, in response to a hypothesized pulse of GOA carbon during late 1995 (Kline 1999) observed in several taxa of fast-growing, lower trophic level organisms (Fig. 1). 24 Kline—Rockfish Trophic Relationships

−19.5

−20.0

−20.5

−21.0

−21.5

−22.0

−22.5 13 E CaTL −23.0 1994B 1995B 1996B 1997B 1994A 1995A 1996A 1997A 1998A

4.2 TL 4.0

3.8

3.6

3.4

3.2

3.0

2.8

2.6 1994B 1995B 1996B 1997B 1994A 1995A 1996A 1997A 1998A

Capelin Northern lampsh Eulachon Northern smoothtongue Glass shrimp Sandlance Juvenile herring Juvenile pollock

Figure 1. Trophic level and lipid normalized stable carbon isotope values 13 (δ C′TL, upper panel) and nitrogen stable isotope value–based trophic levels (TL, lower panel) of forage taxa sample in PWS, 1994-1998. See text for explanation of how years were split. Error bars indicate standard error. Biology, Assessment, and Management of North Pacific Rockfishes 25

Rockfishes prefer rocky, high relief habitats. Western Prince William Sound (W-PWS) is characterized by deep fjords with depths reaching 800 m within 2 km of shore. Depths are <300 m in eastern Prince William Sound (E-PWS), the portion of PWS east of the tanker lanes exclusive of the area near (<30 km) this boundary. Nevertheless, there are loca- tions with rocky habitats in E-PWS with populations of shallow-dwelling rockfish species. A secondary objective was to ascertain if carbon or TL differed qualitatively across PWS. A long-term goal is to recover tagged rockfishes and resample over time for SIA to detect ecosystem shifts. A nonlethal sample would thus be required. The potential for this was tested by tagging and recover- ing rockfishes and by comparing SIA of blood with SIA of muscle tissue. Muscle is typically sampled in most SIA studies after killing the organ- ism being sampled.

Materials and methods Rockfishes and potential prey taxa were sampled incidentally during the Sound Ecosystem Assessment (SEA) project that focused on pink salmon, Pacific herring, and walleye pollock during 1994-1999 (Cooney et al. 2001). Samples were obtained on SEA cruises by seines, trawls, gillnets, and hook and line. Additional rockfish samples from the PWS longline fishery were obtained from Cordova fish processors during SEA. Samples for blood were collected by hook and line or by hand-nets during scuba diving as part of the Oil Spill Recovery Institute Sentinel project in 1999. Rockfish samples were organized according to the fol- lowing three depth-based ecological categories: demersal, pelagic, and slope favored by each species based on their distribution in Alaska (Kramer and O’Connell 1995; Table 3). Rockfishes newly recruited from the plankton have a different appearance from adults and were simply classified as juveniles for the purposes of this study. Rockfish samples were opportunistic and thus not systematic in terms of sampling date or location. Samples came from a range of sites in PWS, which were divided into E-PWS and W-PWS. Data were temporally aggregated to match that of forage data described below. Approximately 1 g of anterior epaxial muscle tissue was collected from each adult rockfish (except for a single ~20 cm long copper rock- fish sampled only for blood), frozen, freeze-dried, then ground to a fine powder. Juvenile rockfish and forage taxa were treated similarly except that the entire organism was ground due to their size (length ≤15 cm). Blood samples taken of fishes collected during July 1999 via caudal vein puncture using non-heparin 3.0 ml syringes were centrifuged, then plasma and cell fractions were frozen and later freeze-dried and agitated to a fine power with a dental amalgamator. Blood and muscle samples were obtained from eight rockfishes (seven dark rockfish and 26 Kline—Rockfish Trophic Relationships SE 0.02 0.08 0.05 0.01 0.05 0.04 0.02 0.02

<0.01 TL 2.9 2.9 3.1 3.3 3.2 3.2 3.5 3.4 3.4 Mean SE 0.07 0.26 0.16 0.01 0.04 0.16 0.14 0.14 0.08 N 15 N values higher than δ 15 δ 11.4 12.0 12.9 12.5 12.6 13.5 13.1 13.1 13.0 Mean

69 1998A

59 1997B

83 288 1997A

92 172 1996B

80 717 1996A 1 Sample periods 16 18 24 27 20 64 175 250 1995B

25 28 22 129 1995A

2 1 11 11 45 86 1994B

7 15 81 69 38 121 1994A 10 18 52 65 39 N 103 124 596 1,839

Scientific name Mallotus villosus Thaleichthys pacificus Pasiphaea pacifica Clupea pallasii Theragra chalcogramma Stenobrachius leucopsarus Leuroglossus schmidti Ammodytes hexapterus Berryteuthis magister Forage taxa Sample sizes and sample timing of forage taxa used for Fig. 1. 1. Fig. taxafor used forage of timing sample and Samplesizes

such as euphausiids and amphipods (Kline 2001). 1999,

N and calculated TL with standard errors confirm forage taxa have ~3. TL Higher TL values likely reflect consuming zooplankton with

15 δ Common name Capelin Eulachon Glass shrimp Juvenile Pacific herring Juvenile walleye pollock Northern lampfish Northern smoothtongue Pacific sand lance Squid Mean Neocalanus Table 2. Table Biology, Assessment, and Management of North Pacific Rockfishes 27 one black rockfish) and from three salmon sampled at the same time, two coho salmon (Oncorhynchus kisutch) and one (O. tshawytscha). One 20 cm long copper rockfish was sampled for blood (1.5 ml) and released live following overnight recovery. Powdered samples were shipped to the University of Alaska Fairbanks Stable Isotope Facility (SIF) where duplicate subsamples of ~0.4 mg were weighed to the nearest 1 µg and loaded into combustion boats for mass spectrometric analysis. A mass spectrometric analy- sis (using either a Europa 20/20 or a Finnegan Delta Plus unit; both equipped with continuous flow sample preparation units) generated the following data: 13C/12C and 15N/14N ratios expressed in standard delta units, δ13C and δ15N, respectively, and %C and %N. The delta nota- tion used to express stable isotope ratios is reported as the per mil (‰) deviation relative to an international standard, air for nitrogen, and Vienna Peedee belemnite (VPDB) for carbon. The conventional delta notation used to express stable isotope ratios relative to international standards is defined by the following expression:

15 13  Rsample  0 δN or δ C = − 1 × 1,000 00 ()1  Rstandard  where R = 15N/14N or 13C/12C. By definition, the isotope standards have 15 delta values of zero, i.e., δ N = 0 for atmospheric N2. Mass spectrometric analysis quality assurance protocols consisted of running laboratory standards before and after groups of five “unknowns” and duplicate analyses of each sample. Samples were run again when duplicate analy- ses differed by more than 0.6 ‰. Normalization for lipid (DeNiro and Epstein 1977) and trophic level effects of δ13C values of fish makes it possible to assess carbon source (Kline 1997, 1999). The method of McConnaughey and McRoy (1979) was used to calculate lipid-normalized 13C/12C while the method of Kline (1997, 1999) was used to normalize for trophic level. Details of these protocols are described in Kline (1997, 1999) and Kline et al. (1998). In general, normalization reduces sources of 13C variability, enabling com- parisons among fishes without the confounding effects of trophic level 13 13 13 13 and lipid content. The expressions δ C, δ C′, δ CTL, or δ C′TL are used to denote 13C/12C abundance in relation to the international standard, nor- malized for lipid content, normalized for trophic level, and normalized 13 13 for lipid content and trophic level, respectively. Whereas δ C′, δ CTL, 13 and δ C′TL is used in accordance to a particular data analysis context, “13C” is used to reflect generic 13C/12C isotopic trends irrespective of normalization. Cross-taxon comparisons of carbon isotope values were 13 made using δ C′TL. 15N/14N-based trophic levels in the northern GOA are accurate to within ±0.3 trophic levels (TL; Kline 2001). Trophic level was determined 28 Kline—Rockfish Trophic Relationships by comparing δ15N values to a reference value (Cabana and Rasmussen 1994.). The δ15N of higher trophic levels were calculated by adding the trophic enrichment factor, 3.4 (Minagawa and Wada 1984; Kline 2001, 2002), to the reference value. Using a single herbivore species as the reference eliminated the compositional (taxonomic and ecologic) vari- ability, and therefore TL uncertainty associated with particulate organic material or bulk zooplankton often used as a TL reference proxy (Kline 1999). The herbivorous copepod Neocalanus cristatus, in diapause, was chosen as the reference herbivore (TL = 2.0) based on observations that their carbon isotope values corresponded with those of PWS fishes (Kline and Pauly 1998; Kline 1999, 2001, 2002). The following formula was used to calculate trophic level:

TL 15NN15 /.3 4 2() 2 i =(δi − δ H ) + 15 15 where TLi is the trophic level of organism i, δ Ni is the mean δ N value 15 15 of organism i, and δ NH is the mean reference herbivore δ N value. The Statview (version 5.0.1; SAS Institute, Inc.) computer program was used for data analyses and to generate all figures. Standard errors were used to qualitatively assess differences among mean values. The forage taxa data (Table 2) have been previously published (Kline 1997, 1999, 2001, 2002), but in different formats. The relatively narrow standard errors of δ15N data and therefore TL (Table 2) provided additional baseline references to that of Neocalanus. Because the inten- tion here is to qualitatively assess the effect of the GOA carbon pulse inferred to have taken place in late 1995 (Kline 1999), data from each year were divided into two parts: A for data collected through August, and B for data collected after August. Data for rockfishes were treated similarly to show temporal trends.

Results and discussion 13 The δ C′TL values of demersal rockfishes either at the species level (Fig. 2) or collectively as a group (Fig. 3) were generally higher than other rockfishes. This is consistent with their consuming more PWS carbon relative to other rockfishes that had more negative values, which sug- 13 gested a dependency, in part, on GOA carbon. Juvenile δ C′TL values 13 approximated those of demersal rockfishes. The δ C′TL values of slope species were the same or more negative than demersals but not as negative as pelagics, suggesting a lower dependence on GOA carbon than pelagics but a greater dependence on GOA carbon compared to 13 demersals (Fig. 3). There were slight differences inδ C′TL values among the demersals for which there were relatively good sample sizes (copper, quillback, and yelloweye) with copper rockfish having the least negative 13 values (Fig. 4). The δ C′TL values of yelloweye and quillback rockfishes shifted concordantly with slight species differences that may reflect Biology, Assessment, and Management of North Pacific Rockfishes 29

−18.5

−19.0

−19.5

E-PWS −20.0 W-PWS

−20.5

−21.0 13 E CaTL

−21.5 Juvenile Dark rocksh Tiger rocksh Black rocksh China rocksh Copper rocksh Quillback rocksh Redstripe rocksh Silvergray rocksh Yelloweye rocksh Shortraker rocksh Greenstripe rocksh Shortspine thornyhead

5.0

4.5

E-PWS W-PWS 4.0

3.5 TL

3.0 Juvenile Dark rocksh Tiger rocksh Black rocksh China rocksh Copper rocksh Quillback rocksh Redstripe rocksh Silvergray rocksh Yelloweye rocksh Shortraker rocksh Greenstripe rocksh Shortspine thornyhead

Figure 2. Trophic level and lipid normalized stable carbon isotope values 13 (δ C′TL, upper panel) and nitrogen stable isotope value–based trophic levels (TL, lower panel) of PWS rockfishes by species by eastern and western PWS. Error bars indicate standard error. 30 Kline—Rockfish Trophic Relationships

–18.5

13 E CaTL –19.0

–19.5

–20.0

–20.5

–21.0

–21.5 1999 1994B 1995B 1994A 1995A

5.0 TL

4.5

4.0

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3.0 1999 1994B 1995B 1994A 1995A

Demersal Pelagic Juvenile Slope

Figure 3. Trophic level and lipid normalized stable carbon isotope values 13 (δ C′TL, upper panel) and nitrogen stable isotope value–based trophic levels (TL, lower panel) of PWS rockfishes by eco-group and year periods as in Fig. 1. Error bars indicate standard error. Biology, Assessment, and Management of North Pacific Rockfishes 31 sample size at some of the sample times (Table 3). Dark rockfish were significantly more negative than the three demersals during late 1995. 13 In late 1995 demersal rockfishes had δ C′TL values falling within the range of values observed the previous 1.5 years, unlike forage taxa that had shifted systematically. The temporal variation of ~2‰ observed in the three demersal rockfish species shown in Fig. 4 was similar to the range observed collectively for demersals and may reflect variation among individuals due to opportunistic feeding. For example, there was a 1 to 2‰ range among potential forage taxa at any one time (Fig. 1). 13 Cross-PWS δ C′TL differences within species (Fig. 2) were small, suggest- ing no apparent strong east-west gradient in effect of GOA carbon sub- 13 sidies in rockfishes. Species δ C′TL differences were greater than these geographic differences (Fig. 2). Overall, pelagic rockfishes had lower 13 δ C′TL values collectively and by species, confirming that the PWS pelagic system depends in part on GOA subsidies (Kline 1999). GOA subsidies are hypothesized to vary over long time spans according inter-decadal variations in oceanic zooplankton populations (Kline 1999). Accordingly, 13 rockfish δ C′TL values are expected to shift over inter-decadal spans while having less short-term variation. Measurable rockfish response to such ecosystem variation may thus have less intrinsic “noise” (response to short-term pulses) compared to lower trophic levels and thus be good sentinel species. Whereas there were no group-specific δ15N, and thus TL, trends among the three groups, there were very large differences among spe- cies (Figs. 2 and 4). Rockfishes occupy TL from slightly greater than TL = 3 or primary carnivore, to about TL = 5 or tertiary carnivore. However, most species were within 0.5 TL of TL = 4 or secondary car- nivore. Comparing the same three species, with good sample sizes as above, suggests an apparent TL difference spanning almost 1 TL (Fig. 4). Shortspine thornyhead and shortraker rockfish with TL ~5 are sug- gested to be among the highest trophic level organisms observed thus far in PWS based on δ15N value (Kline unpubl.). The high TL cannot be explained directly by stomach data (Table 3). However, these are some of the largest rockfishes found in PWS and consequently may be able to prey upon carnivores of TL ~4. Whereas quillback and yelloweye rockfish concordantly shifted their carbon source, differences in TL suggest that their food chains were of different length and thus they did not compete directly, supporting the concept of separate niches. Copper rockfish TL was lower than either 13 and δ C′TL was higher when observed at the same time. Dark rockfish 13 TL may be more similar to that of copper rockfish butδ C′TL was lowest among the four species shown in Fig 4. Thus isotopes of C and N when used together provide two dimensions of ecological niche conferring ecological separation among these congeners. Food chain length of a given species appears to be more conservative than carbon source (Fig. 32 Kline—Rockfish Trophic Relationships * * * * * * * * * * *

Invert. * * * * * * * *

Demersal diet Fishes * * * * *

Invert. * * * * *

Pelagic diet Fishes

1 4 1 1 1 PWS Eastern

Locations

3 4 1 4 1 6 1 1 1 3 3 4 PWS Western

1 8 99 Aug- 1999

1 2 1 1 95 11 13 Nov- 1995B

1 7 9 3 1 95 11 10 12 Oct-

9 95 Jun-

3 1 95 May- 1995A

5 1 1 95 Mar- Sample periods

4 8 7 3 6 94 Sep- 1994B

4 7 94 Jul- 1994A

5 3 94 May-

1 4 1 3 1 7 9 N 11 19 12 32 23 36 Sample sizes and sample timing of rockfish samples by by eco-group. samples rockfish of timing sample and Samplesizes

Common name Eco-group Juvenile Demersal China rockfish Copper rockfish Greenstripe rockfish Quillback rockfish Tiger rockfish Yelloweye rockfish Slope Redstripe rockfish Shortraker rockfish Shortspine thornyhead All slope rockfishes Pelagic Black rockfish Dark rockfish Silvergray rockfish Table 3. Table Number of sampling sites in E-PWS and W-PWS for each species is given. Literature food habit takenabsence from (no symbol) Love et al. of (2002) fish summarized or invertebrate in termsdiet and of presencewhether prey or (*) is demersal or pelagic. Both simplypelagic as fish. and demersal fish indicated in cases when diet was described Biology, Assessment, and Management of North Pacific Rockfishes 33

−18.0

13 E CaTL −18.5

−19.0

−19.5

−20.0

−20.5

−21.0

−21.5 1999 1995B 1994B 1994A 1995A

4.8

TL 4.6

4.4

4.2

4.0

3.8

3.6

3.4 1999 1994B 1995B 1994A 1995A

Copper rocksh Quillback rocksh

Dark rocksh Yelloweye rocksh

Figure 4. Trophic level and lipid normalized stable carbon isotope values 13 (δ C′TL, upper panel) and nitrogen stable isotope value–based trophic levels (TL, lower panel) of four PWS rockfishes by year periods as in Fig. 1. Error bars indicate standard error. 34 Kline—Rockfish Trophic Relationships

Table 4. Blood isotope data from 1999 samples with statistics. See text.

C/N 15 13 13 13 Tissue atoms %N %C δ N δ C δ C′ δ C′TL TL Rockfish (N = 8) Muscle Mean 3.8 14.9 48.3 13.9 –18.9 –19.2 –20.8 3.6 SD 0.1 0.3 1.3 0.3 0.2 0.2 0.2 0.1 Blood plasma Mean 6.1 8.9 46.1 13.3 –21.2 –19.7 –21.1 3.5 SD 0.7 1.2 5.9 0.5 0.3 0.1 0.1 0.1 Blood cells Mean 4.2 13.7 49.1 13.1 –19.5 –19.3 –20.7 3.4 SD 0.3 0.7 1.2 0.5 0.3 0.2 0.2 0.1 Copper rockfish (N = 1) Blood plasma 4.7 10.3 41.6 14.6 –17.4 –16.7 –18.8 3.8 Blood cells 4.0 13.7 47.5 14.6 –17.0 –16.9 –18.5 3.8 Salmon (N = 3) Muscle Mean 4.0 14.1 47.8 13.2 –19.4 –19.4 –20.8 3.4 SD 0.2 0.3 2.2 0.6 1.4 1.2 1.1 0.2 Blood plasma Mean 6.5 9.2 51.7 13.4 –22.1 –20.3 –21.8 3.5 SD 0.4 0.3 1.7 0.4 0.3 0.1 <0.1 0.1 Blood cells Mean 4.0 14.7 50.8 12.4 –20.0 –19.9 –21.1 3.2 SD 0.1 0.1 0.9 0.2 0.1 0.2 0.1 0.1

4). However when aggregated by eco-group, carbon source appears to be more conservative than TL (Fig. 3). TL is thus a property of species whereas carbon source is more of an eco-group property. Blood isotope data were qualitatively similar to muscle tissue (Table 4). The higher C/N ratio of plasma was indicative of relatively high lipid 13 content and when used to correct for lipid effects resulted inδ C′TL val- ues much closer to those of other tissues. Differences among tissue type may reflect differences in turnover time. Each type thus reflects more or less recent versus past feeding (Hobson and Clark 1993). Blood plasma should reflect the most recent feeding. The greatest disparity between 13 blood plasma and muscle δ C′TL of ~1‰, for salmon, may have reflected that much of the muscle tissue was attained while feeding in the GOA 13 prior to entering PWS. Salmon and pelagic rockfish plasma δ C′TL were similar whereas that of the copper rockfish was enriched. Copper 13 rockfish blood in terms of TL and δ C′TLwas similar to copper rockfish sampled four to five years earlier. Dusky rockfish muscle values from 1999 were also comparable to those from 1995 (Fig. 4). The relatively small spatial home range of shallow demersal species such as copper and quillback rockfishes (Richards 1987) (suggested to be 10 km or less in PWS based on ultrasonic pinger data [Kline unpubl.]), their capture Biology, Assessment, and Management of North Pacific Rockfishes 35

ease with scuba, and their ability to be nonlethally sampled for blood, leads to their potential as sentinels for environmental change using SIA of blood. A test that could be performed over long-term (e.g., multi- decade) observations would be to ascertain if ecological niche separa- tion in terms of isotope values was reduced or expanded, a possible result of climate-driven ecosystem shifts.

Acknowledgments This material is based on work supported by the Prince William Sound Oil Spill Recovery Institute, the Exxon Valdez Oil Spill Trustee Council, and the U.S. GLOBEC program through the U.S. National Science Foundation under grant no. 0114560. Any opinions, findings, conclu- sions, and recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation, the Exxon Valdez Oil Spill Trustee Council, or Prince William Sound Oil Spill Recovery Institute. Kim Antonucchi, Bob Hicks, Neal Oppen, Dave Salmon, and John Williams assisted with field sampling and sample processing. Norma Hubenstock and Bruce Barnett performed the mass spectrometric analysis. This manuscript benefited from suggestions made by two anonymous reviewers.

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