Introduction

Sablefish (Anoplopoma fimbria) is an economically valuable commercial groundfish species in the Northeast Pacific Ocean, and its population is assessed annually by the National Oceanic and Atmospheric Administration (NOAA) Alaska Fisheries Science Center (Goethel and others, 2022). Sablefish recruit into the commercially fished population at age-2 and are poorly sampled by most surveys before that age. Previous efforts to better understand age-0 sablefish growth, and possible links to recruitment, documented greatest catches of age-0 sablefish along the seaward edge of the continental shelf in the central and eastern Gulf of Alaska (Sigler and others, 2001). In this region, age-0 sablefish are a main prey species for colonial nesting rhinoceros auklets (Cerorhinca monocerata) and tufted puffins (Fratercula cirrhata) at Middleton Island (fig. 1). Long-term seabird diet sampling conducted at Middleton Island provides annual data on forage fish size and growth over time (Hatch and Sanger, 1992). This information on age-0 sablefish provides one of many metrics that are compiled annually to help evaluate sablefish stock status in the Ecosystem and Socioeconomic Profile (Shotwell and others, 2022).

Methods

Sample collection began in 1978, with collections conducted annually since 1993 (Hatch and Sanger, 1992). Each year during the seabird chick rearing period (late June to August), juvenile sablefish are captured by auklets and puffins foraging offshore. Whole, fresh fish prey are brought back to Middleton Island by seabirds in their “bill loads.” Fish in bill load samples can thus be readily identified, enumerated, and total length (in millimeters [mm]) measured soon after collection. From these measures, taken annually (Arimitsu and Hatch, 2023), various metrics and indices can be calculated, such as catch-per unit effort for each year, frequency of occurrence, and a growth rate index (Hatch and Sanger, 1992).

Because the auklet and puffin chick diet samples are collected over the course of the chick rearing season, an index of age-0 sablefish growth rate is calculated using the relationship between mean fish length and Julian day for each year. From this model, growth (in mm per day) is the slope, and length is the predicted length on the median Julian day = 205 (July 24). Annual anomalies of growth and length are calculated for each variable by subtracting the mean of all years. Data from 1994 were omitted from this analysis because of low sample size (n sablefish = 6) and samples were collected over a short period of time (6 days).

Results

The annual data release (Arimitsu and Hatch, 2023) provides number of sablefish sampled and measured on Middleton Island. In 2022, sablefish made up 20 percent of the total biomass in seabird diet samples (n = 374; June 22–August 21), which is well above the long-term mean (8.7 percent) for Middleton Island samples. The large prevalence in 2022 of sablefish in auklet and puffin chick diets (n sablefish = 284; catch per unit effort = 0.77 fish per sample; frequency of occurrence = 0.35) suggests sablefish were widely available in the seabird foraging area near Middleton Island. The relationship between length and Julian day differs by year (ordinary least squares regression: F = 24.44; df = 61, and 184; R² = 0.85; p < 0.001). During 2022, the average growth index was 1.54 mm per day, which was slightly below the long-term average of 1.88 mm per day. Predicted size on the median sample date was 107 mm, which was 6 mm above the long-term mean of 101 mm (fig. 2).

Age-0 sablefish were larger in 2022 than in 2021. Predicted size (71 mm) on the median sampling date during 2021 was 29 mm below the long-term (1978–2022) mean and only six individual sablefish were sampled by seabirds despite above average diet sampling effort (n samples = 306; long-term average = 180; fig. 2).

Summary

The age-0 sablefish samples obtained from seabird diets at Middleton Island provide information on a critical life stage that is not well sampled by other means. Growth and size of age-0 sablefish in 2022 were close to the means for the entire timeseries. Based on these analyses of size and growth, it is unlikely that age-0 sablefish were large enough during summer of 2021 to be targeted by seabirds as suitable prey for their chicks, which may explain why so few sablefish (n = 6) were sampled that year. In 2019, the growth anomaly was high but size was only moderately above average because fish were small at the beginning of the sampling period and grew unusually fast that year (fig. 2).

Summaries of age-0 sablefish in seabird diets from Middleton Island (Arimitsu and Hatch, 2023) are provided annually to NOAA for the Ecosystem Socioeconomic Profile in the sablefish stock assessment (Shotwell and others, 2022). In the Alaska sablefish stock assessment, the age-0 sablefish growth rate from Middleton Island data are used in the stock assessment as part of a suite of ecosystem indicators, in this case as one of several lower trophic system indicators. Ecosystem indicators are used by NOAA in risk tables or in the stock assessment model to highlight external stressors or factors for managers to consider when determining future Acceptable Biological Catch recommendations for the sablefish stock (Shotwell and others, 2022).