Abstract
Egg surveys are used worldwide for the estimation of spawning stock biomass (SSB) of small pelagic fish species, requiring detailed knowledge about their reproductive biology. In the present study, we revisit the current conceptual framework of teleost fecundity types using Northeast Atlantic (NEA) mackerel (Scomber scombrus) as case study due to conflicting views across different assessment methods. We hypothesized that the herein presented unique time series on protein and lipid content for this stock would help in resolving the long-lasting, intrinsic fecundity type problem. First, we document that the body surplus energy has varied substantially over time, with a significant drop to historically low levels following a stock increase from 2005 to 2015. This fluctuating pattern is in stark contrast to the stable relative fecundity (oocyte g−1 females) measured in connection with the egg surveys. Second, we show that the feeding levels are at the highest during the spawning season. These findings are consistent with an indeterminate fecundity type as opposed to the presently accepted determinate-type classification dating back to the 1990s. Furthermore, we quantify the batch fecundity and find it to be largely constant. Hence, the main reproductive output regulator that is driven by the bioenergetic status should therefore be the number of batches shed. Based on this novel framework for an indeterminate spawner, we provide alternative estimates of relative realized fecundity, which significantly change the egg survey-based SSB indices, reduces the contrast to the other data sources in the mackerel stock assessment (1990–2019), improves the assessment model fit and reduces the uncertainty of the stock size estimate. The presented algorithms and lines of thinking are applicable to other teleosts and may improve the precision and accuracy of the estimation in cases where the annual egg production method is used to assess stock size.
Highlights
Egg production methods (EPMs) are regularly used to assess spawning stock size (SSB) of pelagic fish species like anchovy (Engraulis mordax), sardine (Sardinops sagax), horse mackerels (Trachurus spp.) and Atlantic mackerel (Scomber scombrus) and seeing applications on demersal species like snapper (Pagrus auratus), hake (Merluccius mer luccius), cod (Gadus morhua) and flatfish (Pleuronectes platessa, Solea solea) (Bernal et al, 2012)
Combined with data on body weight, length and stomach fullness during the reproductive cycle as well as associated batch fecundity, we addressed core aspects of this stocks reproductive biology
When protein and lipid resources are available, developing oocytes might continuously be provided from the reservoir of advanced previtellogenic oocytes (PVO4c), quickly developing and going through “an approximate steady FP and FB system” but varying number of batches (NB)
Summary
Egg production methods (EPMs) are regularly used to assess spawning stock size (SSB) of pelagic fish species like anchovy (Engraulis mordax), sardine (Sardinops sagax), horse mackerels (Trachurus spp.) and Atlantic mackerel (Scomber scombrus) and seeing applications on demersal species like snapper (Pagrus auratus), hake (Merluccius mer luccius), cod (Gadus morhua) and flatfish (Pleuronectes platessa, Solea solea) (Bernal et al, 2012). Thereafter, we “zoom-in” on knowledge gaps that have direct implications for the egg survey-based mackerel stock assessment (and fisheries catch advice) This plan is executed by i) analysing the feeding activity during spawning that may indicate an indeterministic reproductive (income breeding) strategy), ii) evalu ating the conception of approximate stability in FB by adding new data, and iii) linking monthly-resolved time series on adult body protein and fat (lipid) content to migration, respiration and realized fecundity. On this basis, we provide an alternative index of FR, calculate the resulting theoretical total egg production (TEP) and SSB, and explore how this alternative mackerel egg survey SSB index performs in the stock assessment. This investigation critically reviews and exemplifies various steps in the process of EPM-based SSBs in the interest of general principles
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