Age-dependent Parameters of the von Bertalanffy, 1938 Growth Model: The Case of Sardinella aurita (Valenciennes, 1847) along the Central Algerian Coast (Southeastern Mediterranean Sea)

Background: Small pelagic teleosts, such as the round sardinella (Sardinella aurita), play a pivotal role in maintaining marine ecosystem architecture and reinforcing regional food security. However, the long-term sustainability of these stocks is increasingly compromised by synergistic pressures from overexploitation and fluctuating environmental factors.

Aims: This study aimed to introduce a modified growth model for short-lived pelagic fish, incorporating age-related variability. The objective was to refine biomass estimations and optimize stock assessment protocols.

Material and Methods: Biological sampling was conducted monthly from January to December 2023 along the central Algerian coast, yielding a total of 1,128 individuals. Two distinct growth formulations were evaluated: the conventional von Bertalanffy Growth Function (VBGF) assuming a constant growth coefficient K, and an age-dependent approach wherein parameter values (K_n, L∞_n) were resolved independently for each identified age cohort.

Results: Comparative analysis revealed two distinct growth scenarios. The classical VBGF scenario (K), characterized by slow growth (K = 0.31 year-1, L∞ = 30.90 cm), projected a maximum lifespan (Tmax) of 10 years of 10 years. Conversely, the age-structured model (Kn) exhibited rapid initial growth (K1 = 2.27 year-1), that progressively decelerated to 0.46 year-1 by age five, projecting a maximum lifespan (Tmax) of 7 years — although only five age classes were resolved from the sampled population due to intense fishing pressure — and asymptotic lengths (L∞n) ranging from 9.35 cm at age one to 25.10 cm at age five. The growth performance index (Φ′) varied from 2.29 to 2.50 across age classes, with a coefficient of variation (CV) of 4%. Total mortality (Z) was estimated at 3.97 year-1. Under the age-structured scenario, natural mortality (M) decreased with age, from 1.10 year-1 at age one to 0.43 year-1 at age five, contrasting with a static baseline M of 0.39 year-1 across all cohorts— while fishing mortality (F) increased with age, from 2.87 year-1 at age one to 3.54 year-1 at age five, compared to a constant F of 3.58 year-1.

Conclusions: The age-dependent model (K_n) provides a more robust and biologically realistic depiction of S. aurita growth life-history dynamics, supporting sustainable management practices that account for rapid early growth and reproductive energy allocation. This data-parsimonious mathematical framework offers a highly practical alternative for stock assessments in data-limited fisheries where complex bioenergetic modeling is logistically infeasible.