BIOTECHNICAL ASPECTS OF AQUACULTURE PRODUCTION OF COMMON MEAGRE Argyrosomus regius IN FLOATING CAGES IN DAKHLA BAY, MOROCCO

Ovo istraživanje je prvi doprinos praćenju hame Argyrosomus regius (Asso y del Rio 1801) koja se uzgaja u plutajućim kavezima usidrenim u zaljevu Dakhla, južno od Maroka. Zootehničkim praćenjem dvaju proizvodnih ciklusa procijenjen je akvakulturni potencijal hame u zaljevu Dakhla. Prva serija sastoji se od 20.000 jedniki mlađi srednje početne težine 4,5±0,13 g i duljine vilice 6±0,19 cm, a zaprimljena je 26. kolovoza 2019. s faktorom kondicije od 2,08 ± 0,35. Druga se serija sastoji od 30.000 jedinki prosječne početne težine 3 ± 0,11 g i prosječne početne duljine 6 ± 0,26 cm, a primljena je 4. lipnja 2020. s faktorom kondicije od 1,39 ± 0,21. Tijekom prvog pokusa koji je trajao 16 mjeseci, zabilježili smo visok potencijal rasta hame. Riba je dosegla 1265±69,2 g prosječne težine i 48±4,32 cm srednje duljine sa specifičnom stopom rasta (SGR) od 0,58±0,11% dan-1 i dnevnim indeksom rasta DGI od 2,45±0,91 g ind-1 dan-1. Omjer konverzije hrane bio je 1,18 (FER=0,84), konačna gustoća bila je 36,94 kg m-3, a faktor kondicije (k) pokazao je srednju vrijednost za cijeli ciklus od 2,18±0,39. Stopa preživljavanja dobivena tijekom prvog ciklusa bila je oko 89,86%. Dok su ribe drugog ciklusa postigle težinu od 1285±69,2 g i veličinu od 47±5,36 cm nakon kraja 18 mjeseci uzgoja, uz SGR od 0,49±0,12% dan-1 i DGI od 2,34±1,35 g ind-1 dan-1, FCR 1,21 (FER=0,83), konačnu gustoću od 28,43 kg m-3 i faktor kondicije od 1,35±0,32. Dobivena stopa preživljenja bila je oko 92,19%. Međutim, prema provedenoj statističkoj analizi, rezultati pokazuju da ne postoji značajna razlika između dvaju proizvodna ciklusa (p>0,05). Ovi rezultati potvrđuju da je hama vrsta od koje se mnogo očekuje u pogledu diverzifikacije akvakulture u zaljevu Dakhla i diljem Maroka.This study is the first contribution to monitoring the rearing of common meagre Argyrosomus regius in floating cages anchored in Dakhla Bay, southern Morocco. The aquaculture potential of common meagre in Dakhla Bay was evaluated by zootechnical monitoring of two production cycles. The first batch consisted of 20,000 fingerlings of 4.5±0.13 g in mean initial weight and 6±0.19 cm in fork length, and was caught on 26 August 2019 under a condition factor of about 2.08 ± 0.35. The second batch consisted of 30,000 individuals of 3±0.11 g in average initial weight and 6±0.26 cm in average initial length, and was caught on 4 June 2020 under a condition factor of about 1.39±0.21. During the first experiment, which lasted 16 months, a high growth potential of common meagre was observed. The fish reached an average weight of 1265±69.2 g and an average length of 48±4.32 cm with a specific growth rate (SGR) of 0.58±0.11% day-1 and a daily growth index DGI of 2.45±0.91 g ind-1 day-1. The feed conversion ratio was 1.18 (FER = 0.84), final density was 36.94 kg m-3 and condition factor (k) recorded a mean value of 2.18±0.39 throughout the cycle. The survival rate at the first harvest was 89.86%. After 18 months of rearing, the fish of the second cycle reached a weight of 1285±69.2 g and a size of 47±5.36 cm, with an SGR of 0.49±0.12 % day-1 and a DGI of 2.34±1.35 g ind-1 day-1, an FCR of 1.21 (FER =0.83), the final density of 28.43 kg m-3 and a condition factor of 1.35±0.32. The survival rate was 92.19%. However, the results of the statistical analysis showed that there was no significant difference between the two production cycles (p > 0.05). These results confirm that common meagre is a very promising species for the diversification of aquaculture in Dakhla Bay and throughout Morocco

Behavioural pattern in changes to diets and food availability during broodstock conditioning of captive clam

In terms of the nutritional needs, the choice of the appropriate phytoplankton strain is the main criterion during bivalves breeding improvement in captivity. Seven nutritional diets based on tree microalgae species (Chaetoceros calcitrans, Tetraselmis suecica and Pavlova lutheri), and seven feeding rations (0%, 1%, 2%, 3%, 4%, 5% and 6%) were used to assess the nutritional pattern of Ruditapes decussatus during conditioning practices. All regimes have shown the highest ingestion rate at low food availability (1% to 3%) with a leaning to diets based on C. Calcitrans followed by T. suecica and P. lutheri respectively.A significant influence of diets quality on absorption efficiency of phytoplankton was observed for both monospecific and bispecific diets (F = 21.78, p =3.2 10-7 *** and F = 250.09, p= 2.2 10-16 ***respectively). Whereas, low influence of food availability on absorption efficiency among all used rations has been revealed by clams reared under mono, bi and trispecific diets respectively (F = 11,19, p= 1.9 10-7 ***; F = 16,63, p= 1.03 10-9 *** and F = 2.78, p=0.0534ns). Consequently, this study could be handled as a standard protocol for conditioning process in furtherance of yield maximizing and cost minimizing during spat production of bivalves in hatcheries