Age and Growth of Quillback Rockfish (Sebastes maliger) at High Latitude

Abstract

Data on age and growth of fishes is critical for effective management; however, growth rates documented in one location may not be representative of other locations, especially for species that occur across wide geographic ranges. Sebastes maliger, quillback rockfish, occur across a broad latitudinal range, but their growth patterns have been quantified only in the southern part of their range. To provide information for S. maliger in the more northern part of its range, we report age and growth patterns derived from otolith analysis from a population collected in southeast Alaskan waters. In southeast Alaska mean annual growth increments for years 1 and 2 range from 60–80 mm, and for ages 6–9 annual growth increments average about 20 mm. From age 10 on average the annual growth increment is about 5 mm. These data can be used in conjunction with harvest data to manage stocks of S. maliger in Alaskan waters.

1. Introduction

Information about species growth rates is crucial for effectively managing fish stocks [1]. Age-based growth models are used to assess most west coast groundfish stocks [2]. However, growth rates of fishes can vary among locations based on variation in water temperature, salinity, and food resource availability [3,4,5,6]. Water temperature is known to affect fish growth rates, but the relationship between temperature and growth can be positive, negative, or indeterminate among different species and locations [1,7,8,9,10,11,12,13]. Growth rates documented in one location may not be representative of other locations, especially for species that occur across wide geographic, and especially, latitudinal ranges [14].

Sebastes maliger, quillback rockfish, range from the Anacapa Passage (off the southern coast of California) to the Gulf of Alaska (Kenai Peninsula). They are a long-lived, benthic-oriented species reaching a maximum age of 95 years and a maximum size of 61 cm (Figure 1). Sebastes maliger constitute an important segment of the commercial rockfish fishery and the recreational charter boat fishery [15]. However, published age and growth information for S. maliger is limited to one study conducted near Puget Sound, Washington, USA, in the southern part of the species’ range [16]. No age and growth studies have been conducted in more northerly parts of S. maliger range, and we assume that temperature and other factors may influence growth of S. maliger at higher latitudes as it does in other species [9,10,12]. To provide information for S. maliger in the more northern part of its range, we report age and growth patterns from a population collected in southeast Alaska.

2. Materials and Methods

We obtained 31 S. maliger (16 male and 15 female) via hook-and-line sampling in Frederick Sound near Admiralty Island, Alaska (57.1365° N, 134.1200° W) in mid-June 2016. We measured standard length and total length to the nearest mm and assigned a unique ID number to each fish. We determined sex and reproductive maturity by inspection of the gonads. We removed sagittal otoliths and stored them dry in small coin envelopes for further analysis.

We determined age at capture by counting presumptive annuli on sagittal otoliths. The term “presumptive annuli” was used because we did not conduct an age validation study; however, otolith annuli have been validated in several other species of rockfishes [8,17]. We prepared otoliths for counting using standard methods for long-lived fishes [18,19]. First, we created a transverse section by grinding down half of each otolith using 500, 800, and 1200 grit silicon-carbide grinding paper on a water-fed polishing wheel, then we burned the transverse section to provide greater contrast for identifying and counting presumptive annuli. We counted presumptive annuli under a dissecting microscope (Leica Wild M10 and Leica Wild M3C; Leica Microsystems Inc. Buffalo Grove, IL USA) at 6X magnification on a black background with reflected light. The burned surface of the otolith was covered with immersion oil to reduce refraction and reflection (Figure 2a). Presumptive annuli were counted independently by two observers. Of the 31 individual otoliths, 6 (19.3%) were estimated to be the same age, 20 (64.5%) were estimated at one year difference, and 5 (16.1%) were estimated at two years difference in age by the two observers (mean difference of counts between observers = 0.97, SE = 0.11). When counts differed between observers, we resolved disparities by mutual inspection.

To estimate the growth curve of S. maliger we combined two methods. First, we plotted size-at-capture versus estimated age to characterize the growth curve for ages 10+. The youngest fish in the sample was 10 years of age, so to estimate the growth curve for ages 1-9 we back-calculated individual size at age for the first nine years for all individuals in the sample from measurements of otolith presumptive annuli. Age estimates were derived from counts of presumptive annuli on the transverse section, whereas annual growth increments were measured on the convex surface of the otolith from photographs of the otolith taken under a Zeiss Axioskop microscope with a Nikon D810 camera. We measured the total radius of each otolith and the radius from the core to each of the first nine presumptive annuli along the longest axis on the convex surface of the otolith (Digimizer, https://www.digimizer.com/download.php). Measurements were scaled (in mm) by reference to a known-size object placed in each of the photographs (Figure 2b). We were not able to consistently resolve growth increments on the otolith from one individual, so the sample size for size at ages 1–9 was 30. Size of otoliths increased with size of fish [20], and the correlation between radius of otolith and total length was significant (t₂₉ = 5.07, p = 0.00002, R-squared = 0.48). We back-calculated lengths at ages 1-9 using a modified Fraser-Lee formula [21]:

L_x = L₀ + ((L_c − L₀)(R_x − R₀))/(R_c − R₀)

(1)

where: L_x = total length at age x; L₀ = total length at birth (4.5 mm; [16]); L_c = total length at capture; R_x = radius of otolith at age x; R_c = radius of otolith at capture; R₀ = radius of otolith at birth (estimated at 0.08 mm).

Lengths-at-age from each fish were averaged for ages 1–9 to produce the early part of the growth curve. We then combined this early trajectory with the best fit line (ordinary least squares regression) from the size-at-age captured plot to create the entire growth curve. To compare differences between sexes in size and age at capture we used a two-sample t-test. In addition, to facilitate comparison of the growth curve generated for S. maliger in southeastern Alaska waters, we fit the data to a von Bertalanffy growth equation [22].

3. Results

In our sample, estimated ages ranged from 10 to 38, and total length at capture ranged from 316 to 474 mm. Growth rates of S. maliger were highest during their first two years of life and decreased as age progressed. Mean annual growth increments for ages 1 and 2 ranged from 60 to 80 mm, and declined to about 20 mm per year at ages 6–9. At ages greater than 10 years the average annual growth increment was about 5 mm (Figure 3). There was no difference between sexes in size at capture (t-test, p = 0.375), or age at capture (t-test, p = 0.1). Parameter values for this sample from the von Bertalanffy model are: k = 0.137, t₀ = −0.78, and L_max = 448 mm (Figure 3).

4. Discussion

Growth of S. maliger in southeast Alaskan waters is similar to patterns and rates identified in the southern part of the species range [16]. Surprisingly, growth of S. maliger does not vary substantially from populations at lower latitudes even though the physical environment varies considerably across this range. This pattern of similar growth curves across wide geographic ranges is consistent with patterns of two other species of rockfish compared across latitude [23], where even populations from distant locations exhibit a high degree of phenotypic similarity in growth rates and patterns [24]. Heterogeneity in growth rates across latitudes is likely, in part, a response to temperature, but may also be due to differences in upwellings, productivity, population density and competition in the local environment [8,25].

Knowing age and size structure specifically for this location allows managers to assess causes and effects of growth rate variation more accurately [25]. This information also allows researchers to detect changes in growth rate or age structure in the S. maliger population caused by climate change or other factors as those changes occur [26,27]. Our age-growth curve for S. maliger provides a baseline for comparison of future studies. Thus, these data are a valuable addition to the management of an economically important species with a latitudinally large and ecologically variable range.

While 31 individuals might be considered a sub-optimal number, we suggest that our analysis for these data is simple. We estimated means for ages 1-9, and we used an ordinary least squares (OLS) regression of size at capture on estimated age to characterize growth at ages 10+. Sample sizes of 30 are more than adequate to estimate means (as evidenced by the narrow 95% confidence intervals), and to estimate a best fit line using OLS regression [28].

5. Conclusions

We have not formally tested hypotheses. Our goal was to only estimate means and best-fit lines and to illustrate the relevance of such information. A formal testing of hypotheses requires the determination of precision generated by the sample size. But often one must weigh the cost of gathering data against the value of greater precision [28]. As such, we encourage researchers the world over to publish growth estimates derived from otolith analysis, even if sample sizes are modest (i.e., 30 or so, individuals). This is especially critical for poorly known species and for isolated locations. Such a collection of published growth data would be of great value for testing synthetic and long-standing hypotheses [24,29], as well as for management of populations [2,3].