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. 2015 Mar 19;10(3):e0120500.
doi: 10.1371/journal.pone.0120500. eCollection 2015.

The potential for spatial distribution indices to signal thresholds in marine fish biomass

Emilie Reuchlin-Hugenholtz  1 , Nancy L Shackell  2 , Jeffrey A Hutchings  3
Affiliations

The potential for spatial distribution indices to signal thresholds in marine fish biomass

Emilie Reuchlin-Hugenholtz et al. PLoS One. .

Abstract

The frequently observed positive relationship between fish population abundance and spatial distribution suggests that changes in distribution can be indicative of trends in abundance. If contractions in spatial distribution precede declines in spawning stock biomass (SSB), spatial distribution reference points could complement the SSB reference points that are commonly used in marine conservation biology and fisheries management. When relevant spatial distribution information is integrated into fisheries management and recovery plans, risks and uncertainties associated with a plan based solely on the SSB criterion would be reduced. To assess the added value of spatial distribution data, we examine the relationship between SSB and four metrics of spatial distribution intended to reflect changes in population range, concentration, and density for 10 demersal populations (9 species) inhabiting the Scotian Shelf, Northwest Atlantic. Our primary purpose is to assess their potential to serve as indices of SSB, using fisheries independent survey data. We find that metrics of density offer the best correlate of spawner biomass. A decline in the frequency of encountering high density areas is associated with, and in a few cases preceded by, rapid declines in SSB in 6 of 10 populations. Density-based indices have considerable potential to serve both as an indicator of SSB and as spatially based reference points in fisheries management.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. North Atlantic Fisheries Organization (NAFO) Subdivisions [46].
The western pollock NAFO subdivisions (4Xopqrs+5Yb+5Zc) are indicated in the pollock science advisory report [47].
Fig 2
Fig 2. Hypothetical concave (type I), linear (type II) and convex (type III) Spatial Distribution Metric (SDM)- Spawning Stock Biomass (SSB) relationship.
These relationships are generated from.SSB = b * SDM c An exponent or c-value >1 will generate a convex relationship, whereas a c-value < 1 will generate a concave relationship. A c-value not significantly different from 1 will generate a linear relationship.
Fig 3
Fig 3. Significant convex relationships between Area Occupied (AO) and Spawning Stock Biomass (SSB) for Western Scotian Shelf (WSS) cod, Eastern Scotian Shelf (ESS) cod and between AO and Spawning Stock Number (SSN) for winter flounder 4X, according to SSB = b * AO c.
Fig 4
Fig 4. Significant convex Low Density Area (LDA)- Spawning Stock Biomass (SSB) relationships for Western Scotian Shelf (WSS) and Eastern Scotian Shelf (ESS) cod and pollock 4Xopqrs+5Yb+5Zc according to SSB = b * LDA c.
The LDAs are inverted, i.e. an increase on the x-axis represents a decrease in the proportion of LDAs.
Fig 5
Fig 5. Exponent c values with 95% confidence interval for different categories of High Density Area (HDA) for six stocks on the Scotian Shelf from significant concave relationships according to SSB = b * HDA c.
Fig 6
Fig 6. Significant positive and concave High Density Area (HDA) 10-Spawning Stock Biomass (SSB) relationships according to SSB = b * (HDA10)c.
Fig 7
Fig 7. High Density Area (HDA) 10-Spawning Stock Biomass (SSB) time series (left) and associated cross correlation functions (right).
These show maximum cross correlation between HDA 10 and SSB at a lag h = -1 for Western Scotian Shelf (WSS) cod and Eastern Scotian Shelf (ESS) cod and h = -6 for American plaice (See Table 4 for the models used to create the time series residuals).
Fig 8
Fig 8. Area Occupied (AO) and Spawning Stock Biomass (SSB) time series (left) and associated cross correlation functions (right).
These show maximum cross correlation between AO and SSB at a lag h = -1 for Western Scotian Shelf (WSS) cod and Eastern Scotian Shelf (ESS) cod and h = -2 for winter flounder (See Table 4 for the models used to create the time series residuals).
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Grants and funding

This work was supported by Prins Bernhard Cultuurfonds (http://www.cultuurfonds.nl/), and Dalhousie University Biology Department (http://www.dal.ca). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.