Length-Based Stock Assessment of the Mackerel Scad, Decapterus macarellus (Cuvier, 1833) From the Tanzanian Coastal Waters

Abstract

This study aimed to conduct a stock assessment of Decapterus macarellus from Tanzanian coastal waters to determine its current stock status and provide appropriate management recommendations. We used a suite of length-based methods, including the Electronic Length Frequency Analysis (ELEFAN) programme to estimate growth parameters and catch curves to evaluate exploitation fishing and mortality parameters; Length-Based Spawning Potential Ratio (LBSPR) to calculate the spawning potential ratio (SPR), length-based indicators (LBI) to assess sustainable fishing levels and length-based reference point (LBRP) to determine the target reference point (RP) for stock spawning biomass (SB). The results indicated that this small pelagic species grows fast, with a growth rate (K) of 0.74 year⁻¹ and a growth performance index (Ф′) of 2.88. However, the stock is currently at risk of overfishing due to high fishing pressure (F/M > 1) and an elevated exploitation rate (E = 0.71), which resulted in a very low SPR (SPR = 0.11) and SB below the target RP. Yield-per-recruit analysis revealed that the current fishing effort (F_curr = 2.57) far exceeded the precautionary limit (F_0.1) and surpassed the fishing mortality rate that maximizes yield per recruit (F_max = 1.73). This excessive effort largely impacted mega-spawners, reducing their proportion in the catch to a critically low percentage (9%). On the basis of these findings, the study recommends reducing the current fishing effort to F_0.1 (0.84) by limiting the number of ring nets, implementing seasonal closures during peak spawning periods and operating in deeper waters (40–200 m) to protect juveniles and minimize mega-spawner capture.