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All issues Volume 60 (2024) Int. J. Lim., 60 (2024) 2 Full HTML
Open Access
Issue
Int. J. Lim.
Volume 60, 2024
Article Number 2
Number of page(s) 8
DOI https://doi.org/10.1051/limn/2024001
Published online 08 March 2024

© L. Royer, Published by EDP Sciences, 2024

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Introduction

Pikes Esox spp. (Actinopterygii, Esociformes) are emblematic fishes because of their strong socioeconomic value for both recreational and commercial fishing (Raat, 1988; Mann, 1996). This genus groups eight species occurring throughout North America and Eurasia (Froese and Pauly, 2023). In France, two species are currently listed: the ubiquitous northern pike Esox lucius Linnaeus, 1758 (Fig. 1) and the newly described Aquitanian pike Esox aquitanicus Denys, Dettai, Persat, Hautecœur, Keith, 2014 (Fig. 2) (Keith et al., 2020). The Aquitanian pike and the allochthonous northern pike co-occur from the Charente to the Adour drainages, because of frequent restocking of the latest since the second part of the twentieth century (Fig. 3) (Denys et al., 2014). Both pike species can be distinguished morphologically by their coat coloration patterns, their snout length as well as lateral scales and vertebrae numbers (Denys et al., 2014; Jeanroy and Denys, 2019). However, if the northern pike is present in the main rivers and lakes with aquatic vegetation, the endemic species seems to be restricted to small tributaries and coastal catchments qualified as poor environment with sandy substrate and few aquatic vegetation (Fig. 3) (Denys, 2017; Keith et al., 2020). E. aquitanicus is currently listed as Vulnerable according to the French IUCN Red List of Threatened Species like E. lucius because of their risk of hybridization (UICN Comité français et al., 2019; Keith et al., 2020). As the Aquitaine pike is then a patrimonial and threatened species, riverine managers need tools to apply a good management and conservation policy (Dudgeon et al., 2005; Maasri et al., 2021).

Length–weight relationships (LWRs) constitute primary knowledges used for fish management and stock assessment. They are necessary for estimating fish biomass from sampled length data, as well as fish growth, and are useful for ecological modelling (Froese, 2006). For that, length L and weight W are related with a mathematical formula W = aLb including two parameters: a coefficient a and the allometric growth parameter b (Keys, 1928). These two parameters are essential to understand the growth of each species. Each pike species has at least one LWR published study (e.g., Kapuscinski et al., 2007; Verreycken et al., 2021; Giannetto et al., 2016; Huo et al., 2017; Parker et al., 2018), except the Aquitanian pike. Irz et al. (2022) published also a LWR of “Esox lucius” from the ASPE database collecting lengths and weights data of French fish species collected by the French Agency of Biodiversity (OFB) since 1980 s, but without distinguishing the two species. Working on data from badly identified taxa can induce some bias on their management (Bortolus, 2008).

The aim of this study was to provide the first LWR for E. aquitanicus based on an extensive sampling throughout its distribution area, to compare it with data of French E. lucius in order to know if there are some differences between both species, and to know if misidentifications in the ASPE database of Irz et al. (2022) have repercussions on their results.

thumbnail Fig. 1

The northern pike Esox lucius: 92 mm from the Vie river at Poiré-sur-Vie (A), 278 mm from the Yser river (B), 770 mm from the Oise river (C); credit photos: Hydrosphere, Fishpass, OFB.
thumbnail Fig. 2

The Aquitanian pike Esox aquitanicus: 111 mm for 9 g from the Ciron stream (A), 480 mm for 816 g from the Courant mort brook (B); 747 mm for 3200 g from the Jalle du Sud stream (C); credit photos: FDAAPPMA47, FDAAPPMA40, FDAAPPMA33.
thumbnail Fig. 3

Distribution area of the two pike species in France (Esox aquitanicus in red, Esox lucius in its native area in blue and introduced in green) (modified and adapted from Keith et al., 2020; Denys, 2017), with examples of typical habitats of both species: the Sèvre nantaise (A) as a large river with many aquatic vegetation and the Ciron stream (B) as watershed with a poor environment (sandy substrate and no aquatic vegetation except stumps); credit photos: G. Denys.

2 Material and methods

Data for Aquitanian pike were collected from 1996 to 2022 during the monitoring of the Departmental Angling Federations and the French Agency of Biodiversity which aim to make the diagnosis of the physical, trophic and physico-chemical state of the environments in order to best adjust the fish management measures. Specimens were caught by electrofishing inventories campaigns before to be released. Populations previously characterized as hybrids or introgressed by Denys et al. (2014, 2018) and Denys (2017) were removed from our dataset as well as the locations already known to have been restocked. Identifications were based on morphological (Denys et al., 2014; Jeanroy and Denys, 2019) or molecular data (Denys et al., 2014, 2018; Denys, 2017). Thus, our dataset is composed by only populations already characterized as pure Aquitanian pike according to morphological or molecular data from previous studies (Denys et al., 2014, 2018; Denys, 2017). Total length (L in cm) was measured to the nearest millimeter and weight (W in g) was determined with a digital balance to an accuracy of 0.1 g. Additional data (n = 338) from locations already knows to shelter Aquitanian pike without any sympatry with the allochtonous species were added from the ASPE database (Irz et al., 2022). The list of locations and measurements are available in Supplementary data 1 and 2.

For northern pike, data were extracted from the ASPE database excluding those from the Adour-Garonne drainages (location codes beginning by “05”) in order to exclude any data corresponding to the Aquitanian pike, and during the period between 1981 to 2017. Only individual measurements were considered. A map synthetizing the sampling is given in Figure 4.

In both datasets, records with weights lower or equal to 1 g were removed considering weighing scales used were not sufficiently accurate to have reliable data for these young offsprings.

Length and weight averages were compared using a U test of Mann–Whitney after having checked a non-normal distribution with a Shapiro–Wilk test (p-value < 2.2 e−16). Length–weight relationships were calculated following the method of Kuriakose (2014) using the equation log10W = log10a + b log10L, where a is the intercept on the Y-axis of the regression curve and b is the regression coefficient (Ricker, 1975). LWRs were calculated for unsexed individuals, as the sex determination is not done by the angling federations nor the French Agency of Biodiversity, because it is not included in their protocol of surveys. The 95% confidence limits of a and b (CL 95%) were also computed (Froese, 2006) for both equations.

Linear regressions and all statistic tests were performed using the R package (R core team, 2022) using lmtest, stats and dplyr packages following Irz (2022). In order to validate the regression model for each dataset, a Rainbow-test was done in order to verify if the residuals mean was equal to 0 (Utts, 1982). Their p-value = 1 and 0.147 for respectively E. aquitanicus and E. lucius indicated a leverage effect. So, the Cook's D distances were then calculated and data exceeding 4/N (N being the number of observations) were removed from the dataset (Bollen and Jackman, 1990).

thumbnail Fig. 4

Locations of both pike species (Esox aquitanicus in red and Esox lucius in blue) from where lengths and weights data came.

3 Results

Data on lengths and weights are provided for a total of 3657 Aquitanian pikes and 5413 northern pikes. The average sizes and weights are respectively 10.7 cm for 20.2 g and 30.2 cm for 339.8 g. Both u-tests of Mann-Whitney on lengths and weights indicates that the average data for E. aquitanicus is lower than those of E. lucius (Wlengths = 1 341 292; Wweights = 1 413 738; p-value < 2.2 e−16 for both datasets). The number of specimens, TL range, parameters a and b with their 95% CL and the correlation coefficient (R2) are reported in Table 1.

The R2 values are respectively equal to 0.982 and 0.988 demonstrating a strong correlation between lengths and weights for both species (Fig. 5).

E. aquitanicus has a lower b parameter than E. lucius (2.960 vs. 2.987) as their range do not overlap (Tab. 1).

Table 1

Descriptive statistics and parameters of total length (TL, in cm) − weight (W, in g) regression for the Aquitanian pike (South West of France) and Northern pike from France; SD means standard deviation.

thumbnail Fig. 5

Lengths (L)–weights (W) regressions of Esox aquitanicus and Esox lucius. Data were log10 transformed.

4 Discussion

LWR of E. aquitanicus and French E. lucius were established from the analysis of 9070 fish.

Aquitanian pike are smaller than northern pike with a maximum size conserved in our dataset of 72 cm for 2.5 kg. Fish sizes are positively correlated with the surface area of habitat and negatively with the temperature (Denys et al., 2016; Kennedy and Rennie, 2023). But Aquitanian pike lives in small tributaries and little coastal basins (Keith et al., 2020), and in southwestern France which is one of the warmer regions in the country (e.g., Parey et al., 2007). However, anglers accounts and data from bibliographical archives described larger specimens: 85 cm for 5 kg (Lagardère, 2020), 107 cm for 9 kg (Glize, 1993) and large fish from 12 to 15 kg (Cahuzac, 2001). So, when the environmental conditions are favourable, Aquitanian pike can reach comparable sizes than northern pike.

The a parameter for the Aquitanian pike is significantly smaller than the one of E. lucius (Tab. 1). Whereas the b parameters are respectively 2.960 and 2.987 in accordance with Carlander (1969) assuming that b normally falls between 2.5 and 3.5. Values of b lower than 3, meaning a “negative” growth, so both pike species become slimmer with increasing length. Comparing both b values, the one E. aquitanicus is lower than E. lucius (Tab. 1). The hypothesis that E. aquitanicus would be more elongated than E. lucius would be false because of its shorter snout and the fewer number of vertebrae (Denys et al., 2014). So, the northern pike would be heavier than the Aquitanian pike for the same given size. Differences between a and b values may be explained by the poor environments where it lives (sandy substrates, few aquatic vegetation, low biomass) conferring a lower primary productivity (Tales et al., 2004). The specific richness is also low (about 5 to 8 co-occurring fish species) (see CGA hydroregion from Santoul et al. (2004) and assemblage type 1 from Park et al. (2006)), and closed to the historical native ichthyofauna community in the Adour-Garonne basin (Keith et al., 2020) with little sized species such as minnows Phoxinus spp, stone loaches Barbatula spp and gudgeons Gobio spp which are not in elevated densities considering the particular habitat (Tales et al., 2004). However within pikes, the growth rate is correlated with the size of the eaten preys (Hart and Connellan, 1984) and the quantity of available food (Kozłowski et al., 2012; Kennedy and Rennie, 2023). A study on ecological traits of the Aquitanian pike is sorely needed to support this hypothesis.

Our a and b parameters for Esox lucius are exactly the same as given by Irz et al. (2022), as well as the R2 (0.987). Their huge dataset (n = 9535) in the ASPE database and removing data after the Cook's D distances calculations has certainly drowned the signal of E. aquitanicus in the dataset. The addition of data from angling federations as well as working on populations correctly identified has allowed to bring new knowledges on the endemic species. Angling federations have useful data and naturalist observations which deserve to be known and used in monitoring, ecological studies and conservations programs (Maasri et al., 2021). Their datasets are then complementary with the database ASPE. However, the data centralization from each departmental angling federation and the access is still a huge challenge.

Managers are encouraged to integrate in their protocol the sex determination. This kind of data would allow to highlight a potential sexual size dimorphism already known within pike species (e.g., Craig, 1996) with female growing faster than males. Nevertheless, these cases are correlated with the availability of food resources and cool temperatures (Kennedy and Rennie, 2023). But the geographical context does not fulfil these conditions.

Finally, our study provides then the first data of LWR for Aquitanian pike which could be implemented in FishBase (Froese and Pauly, 2023). This LWR could be useful for managers for estimating weights from measurements and to know if the environment of a location brings ideal conditions for growth, as well as for other disciplines in biology.

Supplementary Material

Supplementary data 1. List of locations of Esox aquitanicus used in this study.

Supplementary data 2. Measurements of Esox aquitanicus used in this study.

Access here

Acknowledgements

This work was supported by the Muséum national d'Histoire naturelle (MNHN), the Unit PatriNat 2006, the UMR BOREA 8067, the Lorraine University, the French Agency of Biodiversity and the Union of Angling Federations of the Adour-Garonne Basin (UFBAG). This study is included within a global project about the Aquitanian pike BIOESOX funded by the UFBAG and the French Federation of Angling in France (FNPF). We thank all people from the Angling departmental federations and the OFB for providing data.

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Cite this article as: Royer L, Teletchea F, Delavergne S, Lafitte F, Escarpit M, Molina Q, Breugnot E, Cosson E, L'Hostis A, Denys GPJ. 2024. Length–weight relationships of the French pikes Esox spp. (Teleostei, Esocidae). Int. J. Lim. 60: 2:

All Tables

Table 1

Descriptive statistics and parameters of total length (TL, in cm) − weight (W, in g) regression for the Aquitanian pike (South West of France) and Northern pike from France; SD means standard deviation.

In the text

All Figures

thumbnail Fig. 1

The northern pike Esox lucius: 92 mm from the Vie river at Poiré-sur-Vie (A), 278 mm from the Yser river (B), 770 mm from the Oise river (C); credit photos: Hydrosphere, Fishpass, OFB.
In the text
thumbnail Fig. 2

The Aquitanian pike Esox aquitanicus: 111 mm for 9 g from the Ciron stream (A), 480 mm for 816 g from the Courant mort brook (B); 747 mm for 3200 g from the Jalle du Sud stream (C); credit photos: FDAAPPMA47, FDAAPPMA40, FDAAPPMA33.
In the text
thumbnail Fig. 3

Distribution area of the two pike species in France (Esox aquitanicus in red, Esox lucius in its native area in blue and introduced in green) (modified and adapted from Keith et al., 2020; Denys, 2017), with examples of typical habitats of both species: the Sèvre nantaise (A) as a large river with many aquatic vegetation and the Ciron stream (B) as watershed with a poor environment (sandy substrate and no aquatic vegetation except stumps); credit photos: G. Denys.
In the text
thumbnail Fig. 4

Locations of both pike species (Esox aquitanicus in red and Esox lucius in blue) from where lengths and weights data came.
In the text
thumbnail Fig. 5

Lengths (L)–weights (W) regressions of Esox aquitanicus and Esox lucius. Data were log10 transformed.
In the text

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