Distribution of Juvenile Pacific Ocean Perch Sebastes Alutus in the Aleutian Islands in Relation to Benthic Habitat
Distribution of Juvenile Pacific Ocean Perch Sebastes alutus in the Aleutian Islands in Relation to Benthic Habitat
Christopher N. Rooper and Jennifer L. Boldt
Reprinted from the Alaska Fishery Research Bulletin Vol. 11 No. 2, Winter 2005 The Alaska Fisheries Research Bulletin can be found on the World Wide Web at URL: http://www.adfg.state.ak.us/pubs/afrb/afrbhome.php
Alaska Fishery Research Bulletin 11(2):102–112. 2005. Distribution of Juvenile Pacific Ocean Perch Sebastes alutus in the Aleutian Islands • Rooper and Boldt 103 Copyright © 2005 by the Alaska Department of Fish and Game
Distribution of Juvenile Pacific Ocean Perch Sebastes alutus in the Aleutian Islands in Relation to Benthic Habitat
Christopher N. Rooper and Jennifer L. Boldt
ABSTRACT: The habitat of juvenile Pacific ocean perch (POP) Sebastes alutus was identified using data from trawl surveys conducted by the National Marine Fisheries Service and linear modeling techniques. Analyses were car- ried out to evaluate the POP catch per unit effort (CPUE) data in relationship to depth, temperature, and sponge and coral CPUE. Sponge and coral CPUE were positively correlated, while depth and temperature were negatively correlated. Over 96% of the juvenile POP catch was from depths of 76 to 225 m and their CPUE increased with depth, and decreased with increasing temperature. The most important finding of this analysis was that juvenile POP CPUE increased significantly with increasing sponge and coral CPUE. Multiple regression analysis predicting juvenile POP CPUE explained 16% –17% of the CPUE variability using sponge and coral CPUE and either bottom temperature or depth. Juvenile POP were most abundant at sites in the western Aleutian Islands (beyond 170º W longitude), on large underwater banks (Stalemate and Petrel banks), and in passes between islands where currents are strong and prey availability may be higher than surrounding areas. These results suggest sponge and coral have an important role in the early life history of juvenile POP.
INTRODUCTION feed primarily on copepods, graduating to a diet of euphausids as they grow and mature (Paraketsov 1963; Pacific ocean perch Sebastes alutus is an important Carlson and Haight 1976). Adult POP are generally component of Alaska groundfish fisheries, with land- found in high concentrations near the shelf break and ings of 20,500 t (exvessel value of $4.5 million) in in gullies, predominantly in the northern GOA (Major commercial trawl fisheries in Alaska in 2002 (NMFS and Shippen 1970; Lunsford et al. 2001). Adult POP 2004). The catch of Pacific ocean perch (POP) in com- make extensive diel migrations to feed (Skalkin 1964; mercial fisheries occurs primarily in the northern Gulf Lyubimova 1965; Major and Shippen 1970) and have of Alaska (GOA) and Aleutian Islands region, with mi- been found in high densities near the bottom in sea nor, but regionally important catches along the Bering whip beds (Brodeur 2001). The habitat used by ju- Sea shelf break. Landings of POP in Alaska declined venile stage POP has not previously been quantified. substantially in the early 1980s, increased slightly in Submarine observations in southeast Alaska indicate the last half of that decade, and have stabilized since the early juveniles school near rock outcroppings with 1992 (NPFMC 2003). Because POP are long-lived (up epibenthic invertebrates (Carlson and Straty 1981). to 90 years), and slow growing, they may be suscep- These observations, along with information about the tible to overfishing (Chilton and Beamish 1982), yet distribution of other rockfish juveniles (Love et al. there is a critical lack of knowledge about their life 1991), indicate a probable preference for physically history and habitat use. complex habitat by juvenile POP. Pacific ocean perch age of maturity is approxi- One of the major features adding complexity to mately 5–7 years, and they reach a total length of about rocky habitat in the GOA and Aleutian Islands region 200 –250 mm (Paraketsov 1963; Chikuni 1975). Lar- is epibenthic invertebrates such as deepwater sponges val distributions suggest adults spawn near the shelf and corals. These invertebrates are susceptible to dam- break at depths of 200 –250 m in the spring (Lisovenko age by mobile fishing gear (Freese 2001; Krieger 2002) 1964). Pacific ocean perch are ovoviviparous, releas- and are long lived and slow growing (Leys and Lau- ing pelagic larvae that settle after approximately one zon 1998; Stone and Wing 2001; Andrews et al. 2002; year (Carlson and Haight 1976). Early stage juveniles Risk et al. 2002), suggesting recovery times that may
Authors: CHRISTOPHER N. ROOPER is with the Alaska Fisheries Science Center, National Marine Fisheries Service, 7600 Sand Point Way NE, Seattle WA 98115-6349. E-mail: [email protected]. JENNIFER L. BOLDT is with the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) at the Univeristy of Washington. Acknowledgements: We thank Mark Wilkins, Paul Spencer, Bob McConnaughey, David Somerton, Cathy Coon and Gary Stauffer for providing thoughtful input on the analyses and manuscript. Additional thanks to NMFS biologists and charter vessel personnel who collected the data analyzed in this manuscript. Sponsorship: This publication is partially funded by the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement No. NA17RJ1232, Contribution No. 1157.
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Distribution of Juvenile Pacific Ocean Perch Sebastes alutus in the Aleutian Islands • Rooper and Boldt 103
be on the order of decades or centuries. For example, too small scale to apply across broad areas. This study Freese et al. (2001) found that 47% of the sponges in utilized bottom trawl survey data from the Aleutian Is- the path of a trawl were damaged during one pass of lands to evaluate the distribution of POP across a large fishing gear, and no sign of recovery could be seen region encompassing a variety of different habitat types after one year. Although the effects of commercial defined by catch rates of sponge and coral. trawling on sponge and coral are becoming clearer, less clear is how the disturbance of this habitat affects MATERIALS AND METHODS commercial fish species. Pacific ocean perch popula- tion dynamics suggests that juvenile habitat may be a Study area factor limiting adult recruitment in this species (Iles and Beverton 2000). If true, the destruction of critical The Aleutian Islands are a chain of volcanic islands juvenile habitat may reduce the recruitment potential stretching from southwest Alaska across the North for the species. Pacific, dividing the western GOA from the Ber- In general, little is known of the functional rela- ing Sea (Figure 1). The Alaska Coastal Stream and tionships between juvenile POP and their habitat or the Alaska Coastal Current flow westward on the GOA scale at which these relationships occur (Carlson and side of the Aleutian Islands, while on the Bering Haight 1976; Carlson and Straty 1981). Data to help us Sea side the current flows eastward, with extensive determine the functional relationships is lacking, or is transport through passes in the chain from the GOA
168°E 172°E 176°E 180° 176°W 172°W 168°W 164°W
54°N
52°N Stalemate Bank
50°N Dutch Harbor
Amchitka Pass Umiak Pass 48°N
Adak Samalga Pass
46°N # 44°N N
0 100 200 300 400 500 600 700 Kilometers
Figure 1. Map of the Aleutian Islands, showing important features, depth contours, and trawl survey stations in the 1994, 1997 and 2000 trawl surveys. 104 Articles Distribution of Juvenile Pacific Ocean Perch Sebastes alutus in the Aleutian Islands • Rooper and Boldt 105
to the Bering Sea (Stabeno et al. 1999; Stabeno et normality assumptions for analysis, and are hereafter al. 2002). The Aleutian Islands bottom trawl survey defined as sponge and coral CPUE. is conducted along the island chain from longitude 165°W to 170.5°E on the Bering Sea side and from Data analysis longitude 170°W to 170.5°E on the GOA side. The continental shelf is narrow along the Aleutian Is- Raw juvenile POP catch data and the habitat variables lands chain, and depths sampled by the bottom trawl were mapped over the Aleutian Islands region using a survey range from 10 to 550 m. geographic information system (GIS). The combined coral and sponge catch was interpolated to a layer in Trawl survey data the GIS using inverse distance weighting (power = 2, maximum search distance = 20,000 m) with an out- The Alaska Fisheries Science Center has conducted put cell size of 1 ha. The catch of juvenile POP was bottom trawl surveys in the Aleutian Islands region overlaid onto maps of sponge and coral distribution since 1980. Surveys were conducted triennially be- showing the bathymetry of the Aleutian Islands trawl tween 1991 and 2000. For this analysis we used only survey area. Alaska Fisheries Science Center data from the 1994 Data analyses were carried out in a 2-stage process. (n =384), 1997 (n = 439), and 2000 (n = 415) Aleutian Two dependent variables were examined: 1) the pres- Islands trawl surveys, because data collection and ence or absence of juvenile POP, and 2) juvenile POP species identification methods were consistent. A CPUE. The presence–absence data as a function of general description of trawl survey methodology can trawl depth were used to determine the depth distribu- be found in Harrison (1993). Records were only used tion of juvenile POP. Data were then used only from where trawl performance was satisfactory and where tows made in the depth range where over 96% of the the distance fished and net width were recorded. Ad- trawls catching juvenile POP occurred. This eliminated ditionally, trawl survey sites without bottom depth or portions of the survey area in which juvenile POP were temperature data were eliminated. Bottom depth and rarely observed simply because the data were collected temperature were measured using a temperature-depth outside the normal depth range for POP, and allowed us recorder attached to the trawl net and averaged over to analyze habitat variables over an appropriate depth the length of the tow. range for POP. All fish captured during the survey trawls were Correlations among the 4 habitat variables (depth, sorted by species, counted, measured for total length, temperature, sponge CPUE and coral CPUE) were and the total weight and number of each species in computed to determine significant relationships that the catch was determined. In the case of some large existed. Based on this analysis, the CPUE of sponge catches, the total catch was weighed and subsampled. and coral were added for each trawl survey haul to When more than 200 individual POP were captured in facilitate further analysis of juvenile POP CPUE. a tow, POP lengths were subsampled. The POP catch Juvenile POP CPUE was compared to the 3 habitat was divided into juvenile (< 250 mm) and adult stages variables (depth, temperature, sponge and coral CPUE) (>250 mm) based on published literature on POP size using multiple linear regression (Zar 1974). at maturity (Paraketsov 1963; Chikuni 1975). Catch –1 per unit of effort (CPUE, no. × ha ) of juvenile POP RESULTS was calculated using the area swept computed from the average measured net width for each tow multiplied Sponge and coral CPUE was typically high in areas by the distance towed recorded with geographical po- of intermediate depth along the Aleutian chain west sitioning systems. The juvenile POP catch data were of 170°W. The highest CPUE of coral and sponge was transformed for analyses using LN(CPUE +1) to meet found at Petrel Bank (Figure 2). This coincided with normality assumptions, hereafter shortened to juvenile some of the highest juvenile POP CPUE values (Fig- POP CPUE. ure 2). Juvenile POP was highest in areas of elevated Although some invertebrates are identified to spe- coral and sponge CPUE (Figure 2). The distribution of cies during the survey, for this analysis sponges were juvenile POP was strongly influenced by depth (Figure defined as species from the phylum Porifera, while 3). Over 96% of the total juvenile POP catch was be- black corals, gorgonian corals, hydrocorals, cup cor- tween 76 and 225 m. Juvenile POP were caught at only als, soft corals were all grouped as corals. The CPUE 23% of the survey sites in this depth range. So even at of sponge (kg × ha–1) and coral (kg × ha–1) in each tow trawl sites located in a suitable depth range for juvenile was calculated and LN(CPUE +1) transformed to meet POP, they were not caught in over 75% of the tows. 104 Articles Distribution of Juvenile Pacific Ocean Perch Sebastes alutus in the Aleutian Islands • Rooper and Boldt 105 178°W Kilometers 300
180°
200
Petrel Bank 178°E
100
0 176°E # N
174°E
172°E Stalemate Bank
0 1–150 151–350 351–550 551–1,250 0 1–15 16–50 51–150 151–350 351–1,700 170°E Legend CPUE (no/ha) POP Sponge and Coral CPUE (kg/ha) interpolated using inverse distance weighting (grid size =1 ha). interpolated using inverse distance weighting (grid size 53°N 52°N 51°N 50°N 49°N 48°N Figure 2. Map of raw juvenile POP catch data in the Aleutian Islands including raw sponge and coral catch data and depth contours. Sponge and coral catches are 106 Articles Distribution of Juvenile Pacific Ocean Perch Sebastes alutus in the Aleutian Islands • Rooper and Boldt 107 Kilometers 164°W 360
166°W 240
168°W 120
0 170°W Samalga Pass
# N 172°W
174°W
176°W
0 1–15 16–50 51–150 151–350 351–1,700 0 1–10 11–75 76–150 151–350 351–550 551–1,250 178°W Legend Sponge and Coral CPUE (kg/ha) CPUE (no/ha) POP Figure 2. Continued. 52°N 51°N 50°N 49°N 48°N 47°N 106 Articles Distribution of Juvenile Pacific Ocean Perch Sebastes alutus in the Aleutian Islands • Rooper and Boldt 107
103 0.50 1.0
0.45 0.9 169 0.40 0.8
0.35 Proportion of sites 0.7 139 Cumulative frequency 0.30 123 79 0.6
0.25 60 0.5 97 0.20 220 0.4
0.15 0.3
0.10 0.2 Cumulativefrequency of catch 32 29 2 32 0.05 30 28 28 13 18 1 0.1
Proportion of sites where POP were caught Proportionwere POP ofwhere sites 3 7 0.00 0.0 0-25 26–50 51–75 76–100 101–125 126–150 151–175 176–200 201–225 226–250 251–275 276–300 301–325 326–350 351–375 376–400 401–425 426–450 451–475 476–500 Depth (m) Figure 3. Cumulative frequency distribution of juvenile POP CPUE and proportion of trawl survey sites with one or more juvenile POP present. Data are presented in 25-m depth bins, and the number of tows completed in each depth bin are given.
Only data from trawl sites in the suitable depth range juvenile POP CPUE (Figure 6). The resulting model (76 –225 m) were used in further analyses (n = 833). equation was: Most of the habitat variables used in the analyses, Juvenile POP CPUE = 0.990 + 0.305 × S + depth, temperature, sponge CPUE, and coral CPUE, – 0.189 ×T + ε, exhibited significant cross-correlations (Figure 4). Sponge and coral were weakly correlated, although the where T is temperature. The percentage of variability relationship was statistically significant P( < 0.05). Nei- explained in the depth model (17.3%) was only slightly ther sponge nor coral were correlated with temperature larger than the percentage explained in the model in- (P = 0.45 and P = 0.43). Temperature, sponge CPUE, cluding temperature (16.3%). In both these analyses and coral CPUE were all significantly correlated with juvenile POP CPUE was positively correlated with depth. Temperature and coral CPUE decreased with sponge and coral CPUE (Figure 7). increasing depth, while sponge CPUE increased at deeper depths. There was substantial unexplained DISCUSSION variability associated with the depth-invertebrate relationships, although depth and temperature were The separation of adults and juveniles by depth is a tightly linked. common feature among rockfishes (Love et al. 1992; The strong correlation between depth and tem- Murie et al. 1993), as well as other fishes, including perature precluded their simultaneous use in a single flatfishes that utilize nursery areas physically distinct multiple regression, leading to 2 analyses with either from the adult population during juvenile stages temperature or depth combined with sponge and coral (Krygier and Pearcy 1986; Gunderson et al. 1990). CPUE (Tables 1 and 2). When depth was used in the Juvenile POP were only captured in a limited depth multiple regression analysis, it was significantly posi- range during the 1994 –2000 triennial trawl surveys. tively related to juvenile POP CPUE (Figure 5). The Juvenile POP were distributed shallower than adult resulting model equation was: POP, which exhibited highest densities in depths from Juvenile POP CPUE = – 0.363 + 0.295 × S + 190 to 260 m. Juvenile POP catch was unpredictable 0.004×D + ε, even at sites in the preferred depth range, as seen by the high proportion of zero catches at depths from 76 to where S is sponge and coral CPUE, D is depth and ε 225 m (> 0.75). Temperature and depth at each survey is error. When temperature was included in the place site covaried and neither variable could be estimated of depth, it was significantly negatively correlated to independently. 108 Articles Distribution of Juvenile Pacific Ocean Perch Sebastes alutus in the Aleutian Islands • Rooper and Boldt 109 = 0.05 2 R y = 0.08x + 0.05 Sponge CPUE 0.0 2.0 4.0 6.0 8.0 = 0.00 = 0.00 2 2 R R y = 0.03x + 0.03 y = -0.07x + 1.43 C) 0 Temperature ( 3.0 4.0 5.0 6.0 7.0 250 = 0.17 2 = 0.01 R 200 2 R y = -0.01x + 5.12 y = -0.00x + 0.33 150 Depth (m) 100 = 0.03 2 R y = 0.01x + 0.37 50
4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0
relationships for a linear correlation of the plotted variables.
7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0
Coral CPUE Coral
Sponge CPUE Sponge
C) ( Temperature 0 Figure 4. Scatterplot matrix of habitat data collected during the 1994, 1997, and Aleutian 2000 Island trawl surveys. The equations and line represent the best fitting 108 Articles Distribution of Juvenile Pacific Ocean Perch Sebastes alutus in the Aleutian Islands • Rooper and Boldt 109
Table 1. Results of multiple regression analysis of juvenile POP Table 2. Results of multiple regression analysis of juvenile POP CPUE in the Aleutian Islands bottom trawl survey database CPUE in the Aleutian Islands bottom trawl survey database (R2 = 0.174). Degrees of freedom (DF), P values, and F ratios (R2 = 0.163). Degrees of freedom (DF), P values, and F ratios for each factor are shown. The mean squared error (MSE) for each factor are shown. The mean squared error (MSE) and DF for the error term are shown in the bottom row. and DF for the error term are shown in the bottom row. Factors were judged to be significant withP <0.05. Sponge Factors were judged to be significant withP <0.05. Sponge and coral CPUE and juvenile POP CPUE were all LN+1 and coral CPUE and juvenile POP CPUE were all LN+1 transformed prior to analysis. transformed prior to analysis. Source DF F/MSE P Source DF F/MSE P Depth (m) 1 16.72 <0.0001 Temperature (°C) 1 6.81 0.0092 Sponge + coral CPUE 1 156.58 <0.0001 Sponge + coral CPUE 1 154.75 <0.0001 Error 830 1.29 Error 830 1.30
8 y = 0.005x–0.158 7 R2 = 0.029
6
5
4
3 Juvenile POP CPUE 2
1
0 75 95 115 135 155 175 195 215 Depth (m)
Figure 5. Relationship between depth and juvenile POP CPUE during 1994, 1997 and 2000 Aleutian Islands trawl surveys. Data include only depths from 76 to 225 m and juvenile POP CPUE are LN+1 transformed.
The data suggest a significant association of tebrates, primarily Metridium sp. in Southeast Alaska juvenile POP with sponge and coral. This is similar fjords and inlets. Likewise Pearcy et al. (1989) ob- to results reported for juvenile fishes in other areas served schools of small rockfish in rocky, high relief of Alaska, as well as in temperate ecosystems on the areas where the fish swam for cover in anemones and west coast of North America (Carlson and Straty 1981; rock crevices when startled. Small, mostly juvenile, Matthews 1989; Pearcy 1989; Love et al. 1991; Stein rockfishes were found to school at ridge tops in high et al. 1992). Most previous studies have been qualita- relief areas over Heceta Bank, Oregon (Stein et al. tive, typically relying on observations from submarine 1992). Young-of-the-year shallow-water rockfishes dives and remote operated vehicles, and have had dif- in Puget Sound, Washington have also been found to ficulty testing the strength of habitat associations. In exhibit a preference for high relief, hard-bottom areas submersible studies, Carlson and Straty (1981) found (Matthews 1989). In a review of multiple studies, that juvenile rockfish (presumed by the authors to be Love et al. (1991) found that shallow-water rockfishes POP) were found in association with epibenthic inver- generally recruit to hard bottom areas with emergent 110 Articles Distribution of Juvenile Pacific Ocean Perch Sebastes alutus in the Aleutian Islands • Rooper and Boldt 111
8
7 y = –0.202x + 1.435 R2 = 0.008 6
5
4
3
Juvenile POP CPUE Juvenile POP 2
1
0 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 Temperature (°C)
Figure 6. Relationship between temperature and juvenile POP CPUE during 1994, 1997 and 2000 Aleutian Islands trawl surveys. Data include only depths from 76 to 225 m and juvenile POP CPUE are LN+1 transformed.
8 y = 0.339x + 0.145 7 R2 = 0.168
6
5
4
3
Juvenile POP CPUE POP Juvenile 2
1
0 0 1 2 3 4 5 6 7 8 Sponge and coral CPUE
Figure 7. Relationship between sponge and coral CPUE and juvenile POP CPUE during 1994, 1997 and 2000 Aleutian Islands trawl surveys. Data include only depths from 76 to 225 m, and the sponge, coral, and juvenile POP CPUE are all LN+1 transformed. 110 Articles Distribution of Juvenile Pacific Ocean Perch Sebastes alutus in the Aleutian Islands • Rooper and Boldt 111
macrophytes available for shelter. This evidence Aleutian shelf west of 170° (Figure 1). West of Samal- suggests high relief habitat covered with epibenthic ga Pass, a different oceanic regime exists, with more invertebrates or vegetation appears to be vital habitat marine conditions than those east of the pass (Ladd for small juvenile rockfishes. et al. 2005). All Aleutian passes have been found to Considerable variability in the juvenile POP CPUE exhibit high currents, with net flow flowing northward was unexplained by the analyses presented here. Some from the GOA to the Bering Sea (Stabeno et al. 2002). of this variability may be due to sampling biases as- Surface drifter studies have indicated that cyclonic cir- sociated with the sampling gear and survey design. The culation patterns around some of the banks and islands particular bottom trawl used for the Aleutian Islands west of Samalga Pass can occur (Ladd et al. 2005). survey is equipped with 36-cm bobbins on the footrope These circulation features may provide a retention separated with 10-cm rubber disks which keep the net or concentration mechanism for pelagic POP larvae elevated off the seafloor. This reduces the catchability spawned in the central and western Aleutian Islands, of sponges and corals that are small enough to slip which in turn could provide a reasonable explanation under the footrope. Thus, the sponge and coral catch for the occurrence of relatively large abundances of data presented here is probably only an index of the juveniles nearby. Additionally, zooplankton abundance epibenthic invertebrate shelter available to the juve- is especially high at the northern end of passes (Coyle nile POP. Additionally, some hard bottom areas are 2005). These patterns in the oceanography and biologi- not fishable due to obstructions (pinnacles, snags, etc.), cal characteristics may help explain the prevalence of therefore the survey does not cover the entire range of juvenile POP populations around passes and banks in sponge, coral, or juvenile POP distribution. The survey the Aleutian Islands. data, however, do cover an extensive region populated The importance of sponge and coral habitat to ju- by juvenile POP, sponge, and coral, and although un- venile POP is apparent when considering the species certainties associated with sampling bias undoubtedly recruitment dynamics. Their stock-recruit relationship exist, the data set is large and the patterns observed suggests that POP recruitment variability is dampened appear to be robust over the Aleutian Islands. by the concentration of juvenile fish in nursery habi- Additional unexplained variability in juvenile tats (Iles and Beverton 2000). The strong association POP CPUE may be due to habitat characteristics that between juvenile POP and the complex structure of were not measured during the trawl surveys. Only 4 sponge and coral indicates this may be an important variables were available for this analysis of juvenile nursery habitat for the species. Anthropogenic dis- POP distribution, and additional information on preda- turbances to sponges and corals can be significant tion rates, prey availability, current patterns, or other (Freese 2001) and persistent due to slow growth rates, features would have improved the analysis. Previous raising concerns about long-term damage to juvenile work in Alaska has suggested that juvenile rockfish POP nursery habitat. There have been few studies are found in areas of high currents, where little sedi- that document the importance of coral and sponge to ment is deposited (Carlson and Haight 1976). In the commercially important fish species. This study links trawl surveys analyzed for this study, high juvenile areas where the abundance of sponges and corals to POP CPUE commonly occurred over banks such as the catch of juvenile POP, a feature that has long been Stalemate Bank and Petrel Bank, as well as the larger suspected and documented qualitatively (Carlson and deepwater passes common on the central and eastern Straty 1981; Heifetz 2002; Malecha et al. 2005).
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