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Status of Stocks & Multispecies Assessment Program

Developing Maturity Schedules to Improve Stock Assessments for Data-Poor Commercially Important Flatfishes in the Gulf of Alaska

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The goal of this project was to update historical estimates of length and age at maturity for commercially important flatfish species in the Gulf of Alaska (GOA). In order to achieve our objectives, samples were collected at fishery processing plants in Kodiak, Alaska, in 2012 and 2013 through a partnership with the industry group Alaska Groundfish Data Bank (AGDB). Otolith (age structures) and ovary samples were collected from four commercially important flatfish species for the purposes of updating maturity estimates (arrowtooth flounder, Atheresthes stomias; southern rock sole, Lepidopsetta bilineata; northern rock sole, Lepidopsetta polyxystra;, flathead sole, Hippoglossoides elassodon). New data on maturity-at-age can result in changes in the values of the fishing mortality reference points, acceptable biological catch (ABC) and overfishing level (OFL), and the estimate of spawning stock biomass (SSB). Determination of maturity-at-age is crucial in determining the biological productivity of these stocks and the target fishing mortality rate necessary to maintain a healthy reservoir of SSB. These new data will directly enhance stock assessments and fisheries management. Updated maturity estimates are also important to industry to maintain Marine Stewardship Council (MSC) certification of some commercially important flatfish species.

Spawning individuals were defined by those with ovaries containing either hydrated oocytes or ova, with or without post-ovulatory follicles. Reproductive characteristics (notably atresia, or oocyte absorption) were recorded. The break-and-burn method was used for ageing otoliths of each species. Standard quality control methods were used to age specimens, including precision statistics based on testing from a secondary age reader. Data were fitted to a logistic equation to estimate length and age at 50% maturity using generalized linear modeling based on binomial data under the statistical package R. Akaike’s information criterion (AIC) was used as the goodness-of-fit index.

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Figure 1. Histological image of an ovarian cross section from a female southern rock sole showing advanced stages of vitellogenic oocytes as the predominant development stage immediately prior to spawning.

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Figure 2. (From top left) Maturity-at-age ogives for southern rock sole, flathead sole, and arrowtooth flounder, three of the four flatfish species in this study.

Southern rock sole collections were made during the months of April, October, and November (Table 1). Ovaries appeared to be maturing during the spring as evidenced by a relatively large number in later stages of vitellogenesis (Fig. 1). In the fall, spawning was observed in a small percentage of females (hydrated oocytes observed). Of note, our estimate of southern rock sole mature proportion at age was lower than a previous estimate; our estimate was 6.8 years at 50% maturity (Fig. 2), compared to an average age of  9.0 years at 50% maturity from a study conducted from 1996 to 1999 (Stark 2002). Our length at 50% maturity estimate of 35.7 cm was similar to Stark’s (2002) estimate of 34.7 cm (Table 2). 

Flathead sole females were collected during the months from February to April. Collections were conducted during a near-spawning to spawning period for this species. During the months of February and March ovaries from mature females exhibited advanced vitellogenesis. Spawning individuals were observed in April. Our estimates of length and age at 50% maturity were 36.7 cm and 9.2 years, respectively (Fig. 2), respectively. These estimates are slightly larger than Stark’s (2004), whose estimates for 50% of the population were 33.3 cm and 8.7 years.

Arrowtooth flounder were collected over two seasonal periods, in July and during the fall. Based on the histological characteristics, ovaries were primarily developing in July with many females exhibiting some vitellogenesis, and in the fall months, showing more advanced vitellogenesis with a few observed in spawning mode. The maturity estimates for arrowtooth flounder were similar to previous estimates. The length and age at 50% maturity from our study were 48.3 cm and 7.7 years (Fig. 2), respectively. This is in close agreement to Stark’s (2008) study based on GOA collections in 2002 and 2003 that resulted in estimates of length and age at 50% maturity of 46.3 cm and 7.0 years.

The main objective for this cooperative research project was to update estimates of length and age at maturity for four commercially important flatfish species in the GOA. These new estimates will provide updated and critical information for the formulation of reference fishing mortality rates and catch levels (ABCs and OFLs), substantially improving stock assessments for the respective species.  The following conclusions can be made from this study:

  • Collections for three species in this study (southern rock sole, flathead sole, and arrowtooth flounder) were successful for updating estimates of age- and length-specific maturity schedules and providing the best estimates of SSB. This was due to such factors as seasonal timing in sampling, a broad size range of specimens representing both immature and mature fish, and confidence in ageing interpretation. These estimates are considered reliable. The maturity-at-age estimates calculated for these species are now available to update their respective 2015 GOA age-structured stock assessments.

  • The fourth species, northern rock sole, was not successful due to a relatively low sample size resulting in maturity estimates that were inconclusive. More data needs to be collected for this species in the future for more reliable estimates.
Table 1. Number of samples by month by species collected in the Gulf of Alaska (GOA) in 2012 and 2013.

n Length range (cm) Mean size (cm)
Southern rock sole      
     October 2012 78 23−50 38.7
     November 2012 12 25−42 30.5
     April 2013 84 19−60 41.2
     May 2013 21 20−48 30.2
Northern rock sole      
     February 2013 15 28−50 40.9
     April 2013 26 24−56 42.4
     May 2013 21 21−36 27.5
Flathead sole      
     February 2013 74 23−49 38.8
     March 2013 29 31−53 41.3
     April 2013 69 21−49 35.1
Arrowtooth flounder      
     July 2012 52 28−65 47.5
     October 2012 124 21−66 42.7
     November 2012 104 22−80 45.9


Table 2. Estimates of length and age at 50% maturity (L50 or A50) and logistic parameter estimates for flathead sole, arrowtooth flounder and southern rock sole.  
Species Variable Coefficients (α, β) L50 or A50 SE
Southern Rock sole Length (cm) -26.165 0.732 35.73 0.146
  Age -9.427 1.379 6.83 1.631
Flathead sole Length (cm) -22.216 0.606 36.67 0.098
  Age -7.539 0.822 9.18 0.127
Arrowtooth flounder Length (cm) -34.574 0.716 48.32 0.128
  Age -9.102 1.188 7.66 0.170


By Todd TenBrink, Tom Wilderbuer, Ingrid Spies, and Teresa A’mar


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