During routine monitoring of commercial purse seine catches in 2011, 87 fingerling specimens of scombrids were collected in the southern Adriatic Sea. Sequencing of the mitochondrial DNA control region locus inferred that specimens belonged to the Atlantic bluefin tuna, Thunnus thynnus (Linnaeus, 1758) (N = 29), bullet tuna, Auxis rochei (Risso, 1810) (N = 30) and little tunny, Euthynnus alletteratus, Rafinesque, 1810 (N = 28). According to previously published growth parameters, the age of the collected specimens was estimated at approximately 30–40 days, suggesting they might have been spawned in the Adriatic Sea, contrary to the current knowledge. A coupled modelling system with hydrodynamic (ROMS) and individual based model (IBM—Ichthyop) was set up to determine the location of the spawning event. Numerical simulations with the IBM model, both backward and forward in time, indicate commercial tuna cages in the middle Adriatic coastal area as possible spawning location. The two other non-commercial species likely opportunistically use the positive environmental (abiotic and biotic) conditions to spawn in the same area.

The aim of the present study was to investigate the fisheries biology of Scomber japonicus (Houttuyn, 1782) in the Adriatic Sea, to enable implementation of appropriate regulatory measures for fisheries. Reproductive biology and sexual patterns of S. japonicus (chub mackerel) were examined from monthly random samples of commercial catches from purse seine nets (10 mm stretched mesh size) in the eastern Adriatic Sea. The annual mean chub mackerel fork length (FL) was examined from 1998 to 2007. Average fork lengths were approximately 25 cm between 1998 and 2002, decreasing to 22 cm between 2003 and 2007. In addition, sex ratios fluctuated as a function of size and month: females were more abundant in the smaller length classes, as well as before and after spawning; males were more abundant in the larger length classes and during spawning. The overall sex ratio was 0.68; females were significantly predominant. 50% of the chub mackerel population was sexually mature at a FL of 18.3 cm, and both males and females reached sexual maturity at FLs of 20.4 cm and 16.8 cm, respectively. Based on gonad maturity stages, gonad weight and gonadosomatic index, chub mackerel spawned from May to August, with peak spawning in June for both sexes. The mean absolute fecundity was 181 277 ± 62 090 oocytes per ovary. A positive exponential correlation was found between fecundity and fork length, gonad weight, and total chub mackerel weight (r² = 0.640; r² = 0.686 and r² = 0.607, respectively).

The anchovy (Engraulis encrasicolus, Linnaeus, 1758), one of the most important small pelagic fish species in the Adriatic, is currently described as a species that can be considered overfished. From 2013 to 2020, samples of anchovy eggs and larvae were collected through scientific surveys during the summer months. The collected ichthyoplankton data were combined with environmental data (measured satellite sea surface temperature and chlorophyll data, numerically simulated salinity, maps of primary production) to identify anchovy spawning habitats and environmental conditions affecting the anchovy early life stages. For this large dataset, a nonlinear method called Growing Neural Gas Network analysis was used to explain the multiple dependencies between anchovy and the explanatory environmental variables and represent them in 9 patterns called Best Matching Unit (BMU). Obtained values of anchovy early life stages abundances (eggs/m²; larvae/m²) showed a clear negative trend, which was easily observed both in the time series and in the annual spatial distributions. Among all measured environmental parameters that were previously mentioned, salinity showed a significant increase, which can be attributed to the cyclonic phase of the bimodal oscillatory system of the Adriatic and Ionian Seas. The calculated BMUs showed several interesting results that shed new light on previous findings: (a) there is a split between the richer northern and poorer southern parts of the Adriatic in terms of anchovy eggs and larvae abundances, (b) the Kvarner Bay, the west coast of Istria and the area around Dugi otok are consistently rich spawning grounds, (c) decreased abundance in the southern areas is a result of the influence of salinity, (d) an increase in chlorophyll can lead to an increase in egg count, (e) the positive effects of upwelling can be negated by an increase in salinity, (f) increased primary production is followed by increased egg count. Upwelling, as one of the factors that can influence larval and egg abundance by bringing nutrients up from the seafloor, showed increased spatial and temporal variability during the investigated period, which depended on the wind regime. Our analysis showed that neural network analysis can successfully describe the effects and interplay of environmental factors on the abundance of anchovy early life stages.

A sample of 3393 garfish, Belone belone (Linnaeus, 1761) was captured using a seine net between January 2003 and December 2008, along the eastern Adriatic Sea. The range in total length was 20.8–75.4 cm and in weight was 12.21–639.25 g. Length–length equations for converting size measurements (standard length and fork length to total length (TL)) were linear. In the length–weight relationship, positive allometry was established (b = 3.4818). Age, determined from sagittal otoliths, ranged from 1+ to 8+ years. The estimated von Bertalanffy model growth parameters for garfish were L_∞ = 90.3 cm, K = 0.158 year⁻¹, t₀ = −0.109. Otolith weight was endorsed as a possible age predictor, as it showed highly exponential correlation with total garfish length and age (W_o = 0.0012 TL^2.189; W_o = 0.568t^1.486). These findings were used to examine mortality rates and exploitation in order to improve conservation and management of this pelagic species.

Sardine samples from commercial catches obtained by purse seiners operating in Croatian fishing grounds (eastern Adriatic Sea) were collected monthly from March 2017 to February 2018 (excluding January due to a fishing ban). A total of 1085 sardines were analysed. Their total body length and mass ranged from 11.5 to 16.9 cm (mean±SD: 13.9±0.03 cm) and from 11.30 to 54.03 g (mean±SD: 20.31±0.161 g), respectively. Analysis of the length-mass relationship showed positive allometric growth (b=3.3573, r=0.948). Female specimens were predominant (m/f=0.404). According to the monthly gonadosomatic index values, spawning occurred from November to February, which was consistent with previous investigations. The monthly analysis of fat content in the gonads, liver and muscles indicated that the fat content in each studied tissue oscillated seasonally regardless of sex. These alternations were linked to the sardine’s reproductive cycle.