Steep recruitment relationships result from modest changes in egg to recruit mortality rates

https://doi.org/10.1016/j.fishres.2020.105872Get rights and content

Highlights

  • Many fish stocks show surprisingly high recruitment at low spawning stock size.

  • High recruitment can result from rather small density dependent changes in survival.

  • Minor changes in juvenile behavior can be responsible for increased survival.

Abstract

Increasing recruitment per mature adult as density declines is a key factor in being able to sustainably exploit populations and the extent of this increase is closely related to the sustainable exploitation rate of fish stocks. Stock-recruitment estimates typically show surprisingly large increases in egg to recruitment survival as density declines. Because egg to recruit survival rates are often very low, we found that compensatory reductions in total instantaneous egg to recruit mortality rate needed to average just 22–29 % to explain the large increases in egg to recruit survival.

Introduction

Ever since Ricker (Ricker, 1954) introduced the formal analysis of the relationship between spawning stock and subsequent recruitment, there has been interest in how much the recruits per spawner can increase as the abundance of spawners declines. Myers was the first to assemble a large number of spawner-recruit data sets (Myers et al., 1995) and the subsequent analysis (Myers and Barrowman, 1996) showed the pattern across various taxa. Myers’ original data set has now been expanded to include over 1000 fish stocks (Ricard et al., 2012). Most of these data sets show evidence of relatively strong compensatory increase in pre-recruit survival rates at lower stock sizes. When this increase is measured by the ratio of juvenile survival rates at low stock sizes to survival rates at unfished stock sizes, survival rates are often seen to improve by factors of 20 or more. Indeed, that dramatic improvement in survival is the most common ecological basis for sustainable fisheries, though compensatory improvements in body growth and natural morality rates of older fish may also play some role. Rose et al. (Rose et al., 2001) showed that any species that can survive at high rates from egg to recruitment must show dramatic compensation because in the unfished state, total recruitment must be equal to natural mortality of the spawning population.

At first glance, it seems unreasonable or unlikely that juvenile fish could survive at so much higher rates when they are at low densities. In this note, we show that relatively small changes in juvenile survival rates are actually required to explain this and use a range of case examples to demonstrate this effect.

Section snippets

Material and methods

In an unfished population that is stable, the mean annual recruitment rate per spawner must equal the average annual mortality rate of spawners. Given mean fecundity f per spawner and annual spawner mortality rate m, this balance relationship can be expressed in terms of the total pre-recruit mortality rate Mo asfe−Mo = m

This relationship implies that we can calculate Mo given estimates of f and m asMo = -ln(m/f) = ln(f)-ln(m)

For semelparous species like Pacific salmon, m = 1 and Mo is given

Results

For the sample of species in Table 1, fecundities per female vary by three orders of magnitude and mean compensation ratios vary by one order of magnitude; from near four to over 40. As expected from other studies like Goodwin et al., (Goodwin et al., 2006) there is only weak positive covariation between fecundity and CR (Fig. 1), and high fecundity is certainly not a guarantee of strong recruitment compensation. For species with data from multiple stocks, there is considerable intraspecific

Discussion

Much of the post-larval mortality risk (as measured by Mo-Megg) likely occurs soon after hatching when juveniles are small and need to grow and find refuge from predators. The pattern in Fig. 2 can be explained by a behavior hypothesis that when juvenile densities are very low (e.g. when stock size has been severely reduced by fishing), juveniles are spending approximately 20–40 % less time in relatively risky activities. When juveniles are spatially concentrated in the first place (e.g. in

Funding

This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.

Data availability statement

All data used in this analysis are available from published papers that are cited or are shown in Supplemental Table 1.

CRediT authorship contribution statement

Ray Hilborn: Conceptualization, Investigation, Writing - original draft, Writing - review & editing. Carl J. Walters: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Writing - original draft, Writing - review & editing.

Declaration of Competing Interest

The authors report no declarations of interest.

References (21)

  • X. He et al.

    Effects of recruitment variability and fishing history on estimation of stock-recruitment relationships: two case studies from US West Coast fisheries

    Fish. Res.

    (2019)
  • M.J. Bradford

    Comparative review of Pacific salmon survival rates

    Can. J. Fish. Aquat. Sci.

    (1995)
  • M.J. Bradford et al.

    Reference points for coho salmon (Oncorhynchus kisutch) harvest rates and escapement goals based on freshwater production

    Can. J. Fish. Aquat. Sci.

    (2000)
  • B. Dorner et al.

    Historical trends in productivity of 120 Pacific pink, chum, and sockeye salmon stocks reconstructed by using a Kalman filter

    Can. J. Fish. Aquat. Sci.

    (2008)
  • R.E. Forrest et al.

    Estimating thresholds to optimal harvest rate for long-lived, low-fecundity sharks accounting for selectivity and density dependence in recruitment

    Can. J. Fish. Aquat. Sci.

    (2009)
  • R.E. Forrest et al.

    Hierarchical Bayesian estimation of recruitment parameters and reference points for Pacific rockfishes (Sebastes spp.) under alternative assumptions about the stock–recruit function

    Can. J. Fish. Aquat. Sci.

    (2010)
  • R. Froese et al.

    Minimizing the impact of fishing

    Fish Fish.

    (2016)
  • N.B. Goodwin et al.

    Life history correlates of density-dependent recruitment in marine fishes

    Can. J. Fish. Aquat. Sci.

    (2006)
  • C.P. Goodyear

    Assessing the impact of power plant mortality on the compensatory reserve of fish populations

  • E.D. Houde

    Comparative growth, mortality, and energetics of marine fish larvae: temperature and implied latitudinal effects

    Fish. Bull.

    (1989)
There are more references available in the full text version of this article.

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