Interannual variability in early life phenology is driven by climate and oceanic processes in two NE Atlantic flatfishes

Abstract

Early life phenology is a crucial factor for population dynamics in a climate change scenario. As such, understanding how the early life cycle of marine fishes is influenced by key oceanic and climate drivers is of chief importance for sustainable fisheries. This study documents interannual changes in early life phenology of two commercial flatfishes: European flounder (Platichthys flesus) and common sole (Solea solea) from 2010 to 2015 based on otolith microstructure. Using GAMs, we looked for correlations of the North Atlantic Oscillation (NAO), Eastern Atlantic pattern (EA), sea surface temperature (SST), chlorophyl a concentration (Chla) and upwelling (Ui) variation with the onset of hatch, metamorphosis, and benthic settlement day. We concluded that higher SST, more intensive upwelling, and EA were coincident with a later the onset of each stage, while increasing NAO induces an earlier onset of each stage. Although similar to S. solea, P. flesus showed a more complex interaction with the environmental drivers, most possibly because it is at its southern limit of its distribution. Our results highlight the complexity of the relationship between climate conditions and fish early life history, particularly those with complex life cycles that include migrations between coastal areas and estuaries.

Figure 1

Geographical location of the Portuguese Atlantic coast (A) and the sampling stations in the Mondego estuary (B), denoted by the letters M, N1, N2, S1 and S2. Maps were created using QGIS 3.10.10 software (https://qgis.org).

Figure 2

0-group European flounder (Platichthys flesus) and common sole (Solea solea) sagittae otoliths at 50× magnifications, highlighting the daily growth increments and the early ontogenic development stages: L—larval (pelagic), M—metamorphosis, J—juvenile (benthic). Circles indicate the respective onset day for each stage.

Figure 3

Monthly values for the selected environmental factors in the Portuguese Atlantic coast (2010–2015): (A) Sea Surface Temperature (SST; °C); (B) Chlorophyll a (Chla; mg L⁻¹); (C) North Atlantic Oscillation index (NAO); (D) East Atlantic Pattern (EA); (E) Upwelling index (Ui; m³ s⁻¹ km⁻¹).

Figure 4

Linear regression between total length (cm) and age (days) for 0-group juvenile P. flesus (A) and S. solea (B) between 2010 and 2015.

Figure 5

Hatch, metamorphosis, and settlement day distribution boxplots for (A) Platichthys flesus and (B) Solea solea from 2010 to 2015 (in Julian days). The secondary Y-axis shows the corresponding month. The boxplot horizontal thick lines represent the median value and each box the interquartile range from 25 to 75%. The bars represent the largest value within 1.5 times the interquartile range above the 75th percentile and the smallest below the 25th percentile respectively, and the outliers are represented by the black dots. The secondary Y-axis represent the respective month.

Figure 6

Generalized Additive Models (GAM) smoother response curves of the environmental drivers explaining the variation in Platichthys flesus hatch, metamorphosis, and settlement day for the period between 2010 and 2015. SST—Sea Surface Temperature (°C); Ui—Upwelling index (m³ s⁻¹ km⁻¹); NAO—North Atlantic Oscillation; EA—Eastern Atlantic Pattern. Black dots depict the partial residuals for each term. The uncertainty bands denote the 95% confidence intervals for each term.

Figure 7

Generalized Additive Models (GAM) smoother response curves of the environmental drivers explaining the variation in Solea solea hatch, metamorphosis, and settlement day for the period between 2010 and 2015. SST—Sea Surface Temperature (°C); NAO—North Atlantic Oscillation. Black dots depict the partial residuals for each term. The uncertainty bands denote the 95% confidence intervals for each term.