Mitigating bycatch in Mediterranean trammel net fisheries using species-specific gear modifications.

Small-scale fisheries (SSF) use static gear which are thought to interact with marine ecosystems more benignly than towed gear. Despite this, trammel nets, one of the most extensively used type of fishing gear in the Mediterranean SSF, generate large amounts of discards, which can account for 25% or more of the captured biomass. Discarded organisms may include endangered or threatened species such as elasmobranchs, as well as non-commercial invertebrates that damage fishing gear or cause disentanglement delays. We evaluated various trammel-net gear modifications, including (i) the use of a guarding net attached to the footrope, (ii) increasing the length of the rigging twine between the footrope and the netting panel, and (iii) decreasing the mesh size of the outer panels. The last two modifications were successful in lowering captures of the marbled electric ray Torpedo marmorata, which is commonly discarded in the study area. Both sorts of modifications are relatively simple, their manufacturing does not represent an added cost to implement, and most importantly they do not negatively affect the catch of the target species. The current study shows that prior evaluation of the discard profile of distinct métiers is essential to accomplish species-specific gear modifications and underlines the importance of collaboration among scientists, fishers and gear manufacturers.

Oogonial proliferation and early oocyte dynamics during the reproductive cycle of two Clupeiform fish species.

Although oogonial proliferation continues in mature females in most teleosts, its dynamics and the transformation of oogonia to early meiotic oocytes during the reproductive cycle have received little attention. In the present study, early oogenesis was examined throughout the reproductive cycle in two Clupeiform fishes, the Mediterranean sardine, Sardina pilchardus, and the European anchovy, Engraulis encrasicolus. Observations using confocal laser scanning microscopy (CLSM) provided extensive information on markers of oogonial proliferation (mitotic divisions, oogonia nests) and meiotic prophase I divisions of oocyte nests (leptotene, zygotene, pachytene, diplotene) in ovaries of different reproductive phases. In sardine, oogonial proliferation persisted throughout the entire reproductive cycle, whereas in anchovy, it was more pronounced prior to (developing ovaries) and after (resting ovaries) the spawning period. Anchovy exhibited a higher rate of meiotic activity in developing ovaries, whereas sardine exhibited a higher rate in resting ovaries. The observed differences between the two species can potentially be attributed to different seasonal patterns of energy allocation to reproduction and the synchronization between feeding and the spawning season.

Tracking oocyte development and the timing of skipped spawning for north-east Arctic haddock (Melanogrammus aeglefinus).

The present study tracked oocyte development over 9 months and noted incidences of 'skipping', i.e., adults terminating their upcoming reproductive cycle, in field-caught north-east Arctic (NEA) haddock (Melanogrammus aeglefinus), currently the largest stock of this species. Applications of advanced image and histological techniques revealed the presence of cortical alveoli oocytes (CAO), which prevailed as the most advanced oocyte phase for 4-5 months. This new finding of an extended and early appearance of CAOs in this gadoid was supported by that vitellogenesis first started to appear 3 months later. The subsequent oocyte growth trajectories indicated that larger individuals [total length (TL) = 70 cm] typically spawn in the order of 3 weeks earlier than the smaller ones (TL = 40 cm). The spawning season appeared stretched over about 3 months. The majority of skipping females arrested oocyte growth at the CAO phase followed by atretic reabsorption. Compared to those individuals maturing for the spawning season, 'skippers' generally exhibited lower body condition, characterized also by relatively lower liver sizes at the time of the main spawning season. This study demonstrated well-developed skipping dynamics, but also that the CAO period, i.e., when skipping takes place, may be exceedingly long in this commercially valuable gadoid and that its reproductive cycle in many ways deviates from that of the data-rich, sympatric NEA cod (Gadus morhua).

Balbiani body formation and cytoplasmic zonation during early oocyte development in two Clupeiform fishes.

The Balbiani body (Bb) was examined in primary growth phase oocytes for the first time in two clupeoid fish species, the Mediterranean sardine, Sardina pilchardus, and the European anchovy, Engraulis encrasicolus, which belong to different families, Clupeidae and Engraulidae, respectively. Cytoplasmic morphological changes of early secondary growth oocytes were also investigated using confocal laser scanning microscopy, light and transmission electron microscopy. The ultrastructural observations showed that the two species develop a distinct spherical Bb. However, differences in the cytoplasm, mainly in the perinuclear area, were observed. Briefly, in sardine the Bb coexists with a thick perinuclear ring containing mitochondria, nuage, endoplasmic reticulum and small vesicles, while in anchovy this perinuclear ring is thinner, consisting of complexes of nuage and mitochondria. After the disassembly of the Bb, a prominent cytoplasmic zonation develops in the secondary growth oocytes of sardine and anchovy, although with different organelle distribution between the two species. Sardine oocytes exhibit a thick zone of endoplasmic reticulum around the nucleus, whereas in those of anchovy, a thick mitochondria-rich ring surrounding the nucleus was observed. The cytoplasmic characteristics, such as the perinuclear ring in primary oocytes in sardine and the mitochondria-rich ring of early secondary oocytes in anchovy, are also discernible in histological sections by standard procedures and could thus be used as indicators of maturity or imminent spawning period in routine light microscopy observations, providing a valuable tool for applied fisheries biology.

Ultrastructural changes in mitochondria during oogenesis in two phylogenetically close fish species.

The ultrastructure of oocyte mitochondria and their contribution to the endogenous autosynthesis of the yolk was investigated in two clupeoid species, the Mediterranean sardine, Sardina pilchardus, and the European anchovy, Engraulis encrasicolus. The structure and abundance of mitochondria differ in secondary growth oocytes of the two species, whereas they are similar in chromatin nucleolus and primary growth oocytes. Sardine oocytes show a higher percentage of mitochondria in the cytoplasm as they develop. However, the individual size of each mitochondrion decreases, becoming smaller than those observed in anchovy oocytes. The volume fraction of cristae in mitochondria of sardine oocytes gradually increased throughout the oocyte developmental phases up to the early secondary growth phase and then slightly decreased during the mid-secondary growth phase. In the cytoplasm of early secondary growth oocytes of anchovy, the percentage of mitochondria is larger than in mid-secondary growth oocytes. As oocytes develop, the size of mitochondria diminishes as well. In contrast to the volume fraction of cristae in mitochondria of sardine oocytes, the volume fraction of cristae in anchovy was decreased in early secondary growth oocytes and then it was increased during the mid-secondary growth phase. As a result, based on both cytoplasmic dynamics of each species and mitochondrial alterations, it was assumed that mitochondria in sardine play a role in the formation of yolk granules, whereas mitochondria in anchovy play a role in the lipid synthesis pathway. Both species showed exogenous heterosynthesis of yolk, through the process of pinocytosis in the zona radiata of oocytes.

Using clustering algorithms for identification of fish oocyte cohorts based on the characteristics of cytoplasmic structures.

In batch spawning fish, secondary growth oocytes (SGO) are recruited and spawned in successive cohorts, and multiple cohorts co-occur in spawning-capable females. So far, histological features such as the prevalence of cortical alveoli or yolk granules are conservatively used to distinguish oocytes in different developmental stages which do not necessarily correspond to different cohorts. In this way, valuable information about spawning dynamics remains unseen and consequently misleading conclusions might be drawn, especially for species with high spawning rates and increased overlapping among oocyte cohorts. We introduce a new method for grouping oocytes into different cohorts based on the application of the K-means clustering algorithm on the characteristics of cytoplasmic structures, such as the varying size and intensity of cortical alveoli and yolk granules in oocytes of different development. The method allowed the grouping of oocytes without the need of using oocyte diameter, and thus, a crucial histological bias dealing with the cutting angle and the orientation of reference points (e.g. nucleus) has been overcome. Using sardine, Sardina pilchardus, as a case study, the separation of cohorts provided new insight into the ovarian dynamics, indentifying successive recruitment of up to five oocyte cohorts between SGO recruitment and spawning. These results verified previous histological indications of the number of cohorts in sardine. Altogether, this method represents an improved tool to study species with complex ovarian dynamics.

Development of a new 'ultrametric' method for assessing spawning progression in female teleost serial spawners.

The collection and presentation of accurate reproductive data from wild fish has historically been somewhat problematic, especially for serially spawning species. Therefore, the aim of the current study was to develop a novel method of assessing female spawning status that is robust to variation in oocyte dynamics between specimens. Atlantic cod (Barents Sea stock) were used to develop the new 'ultrametric' method, that is based on the progressive depletion of the vitellogenic oocyte pool relative to the rather constant previtellogenic oocyte (PVO) pool. Fish were subsequently partitioned into one of four categories that accurately reflected changes in their oocyte size frequency distribution characteristics and gonadosomatic index throughout spawning. The ultrametric method overcomes difficulties associated with presence of bimodal oocyte distributions, oocyte tails, lack of clear hiatus region, and presence of free ova, and can be implemented at a single sampling point. Much of the workflow is fully automated, and the technique may circumvent the need for histological analysis depending on the desired outcome. The ultrametric method differs from the traditional autodiametric method in that PVOs can be separated by ultrasonication and then enumerated, and ovarian homogeneity is not a mandatory requirement per se. The method is designed for determinate spawners but might be extended to include indeterminate spawners.

Zinc Uptake, Photosynthetic Efficiency and Oxidative Stress in the Seagrass Cymodocea nodosa Exposed to ZnO Nanoparticles.

We characterized zinc oxide nanoparticles (ZnO NPs) by dynamic light scattering (DLS) measurements, and transmission electron microscopy (TEM), while we evaluated photosystem II (PSII) responses, Zn uptake kinetics, and hydrogen peroxide (H2O2) accumulation, in C. nodosa exposed to 5 mg L-1 and 10 mg L-1 ZnO NPs for 4 h, 12 h, 24 h, 48 h and 72 h. Four h after exposure to 10 mg L-1 ZnO NPs, we noticed a disturbance of PSII functioning that became more severe after 12 h. However, after a 24 h exposure to 10 mg L-1 ZnO NPs, we observed a hormetic response, with both time and dose as the basal stress levels needed for induction of the adaptive response. This was achieved through the reduced plastoquinone (PQ) pool, at a 12 h exposure, which mediated the generation of chloroplastic H2O2; acting as a fast acclimation signaling molecule. Nevertheless, longer treatment (48 h and 72 h) resulted in decreasing the photoprotective mechanism to dissipate excess energy as heat (NPQ) and increasing the quantum yield of non-regulated energy loss (ΦNO). This increased the formation of singlet oxygen (1O2), and decreased the fraction of open reaction centers, mostly after a 72-h exposure at 10 mg L-1 ZnO NPs due to increased Zn uptake compared to 5 mg L-1.