Biochemical composition and fatty acid profile of gonads from wild and cultured shortfin corvina ( Cynoscion parvipinnis ) during the early maturation stage

Cynoscion parvipinnis is an important fisheries resource of the Gulf of California used locally for human consumption. Its aquaculture potential has recently been demonstrated, and cultured organisms are already available. In this study, the gonadal developmental stage, proximate composition, fatty acid profile and other biochemical and biological indices of wild and cultured fish were characterized. A total of 80 fish, 30 cultured (15 female and 15 male) and 50 wild (31 female and 19 male), captured in November at Santa Rosa, Sonora, Mexico, were analyzed. Histological analyses of the gonads showed that 100% of the wild and cultured fish were at an early maturation stage. Wild fish had greater body weight, total length and gonadosomatic index. Cultured female fish (19.64%) possessed a slightly higher protein content in their gonads than wild females (17.11%), whereas males had similar values (14.00 and 14.83%). Cultured females (10.85%) and males (20.48%) had a significantly greater crude fat content than wild fish (female: 2.05%, male: 11.05%). Palmitic acid was the most abundant fatty acid (FA), ranging from 128.58-164.84 mg of 16:0 g-1 of gonad wet weight. After 16:0, highly unsaturated FAs like DHA, ARA and EPA were the major FA in gonadal tissue; n-3 FAs were quantitatively higher than n-6 FAs, while the n-3/n-6 ratios ranged from 2.08-2.81. In general, the biochemical composition of the gonad of wild and cultured organisms at an early maturation stage was quite similar. These data may serve as indicators of dietary requirements for a maturation diet for broodstock culture.


INTRODUCTION
The shortfin corvina (Cynoscion parvipinnis) is a mid-sized sciaenid fish, measuring up to 60 cm in total length, that inhabits coastal sandy bottoms of the Gulf of California, Mexico [1].It is valued as a food fish, and it is also popular among recreational fishing enthusiasts.Commercial fishery landings of C. parvipinnis and other members of the genus Cynoscion, such as C. othonopterus, all of them known as corvinas, collectively reached 5961 MT in the Mexican Pacific coast in 2013 [2].Reports indicate that over the past years the shortfin corvina has been overexploited, which along with habitat modification, has led to a decline in their natural populations [3].Recently, there has been growing interest in the shortfin corvina as a species suitable for aquaculture.Due to its high-quality fillet, aquacultural production of shortfin corvina would enhance food fish supply in a locally well-established market, and at the same time, it could alleviate fishing pressure.Based on juvenile fish obtained from wild broodstock acclimated to and spawned in captivity, promising results have been obtained in comparative growth trials conducted in northwest Mexico, which have elucidated various nutritional aspects of this species [4,5].However, continuously controlled reproduction over successive generations is a requisite to establish any fish species as a solid candidate for aquaculture.A first step towards the achievement of this goal is gathering information on its reproductive biology.Typically, male and female gonads are examined and classified into developmental stages, either by macroscopic visual cues, such as size and color [6], or by microscopic, histological preparations, which are generally preferred because they are considered more accurate [7].This information helps determine useful parameters in the management of fisheries resources, e.g., the length of the reproductive season, age/size at first maturity, spawning frequency, etc. [8].In addition, for aquacultural purposes, it aids in the timing of induced spawning [9].
The composition of fish gonads, in terms of the fatty acid profile, is another important aspect for successful reproduction of fish in captivity.As opposed to saturated and monounsaturated fatty acids, which are a preferred source of catabolic energy, long-chain, polyunsaturated and highly unsaturated fatty acids (PUFA and HUFA, respectively) are usually transferred to fish oocytes [10].The HUFA docosahexaenoic acid (22:6n-3; DHA), eicosapentaenoic acid (20:5n-3; EPA), and arachidonic acid (20:4n-6; ARA) play roles as constituents of neural tissues, are precursors of hormone-like molecules, and are involved in the development of the embryo's immune system and hatching [11,12].Therefore, these fatty acids are usually present in relatively high amount in gonads of saltwater fish [13,14].The relative concentration of some fatty acids, e.g., DHA, could be indicative of gonad maturation, an aspect that deserves to be investigated.On the other hand, in the steroid hormone-driven process of ovarian maturation, 17β-estradiol (E 2 ) stimulates the synthesis of vitellogenin in the liver of female fish, which is then incorporated into the developing oocytes [15][16][17].In some instances, gonad size/weight has been found to correspond with the concentration of E 2 in plasma [18][19][20], suggesting that E 2 may be used as an indicator of final oocyte maturation in female fish [21,22].Two additional biochemical components relevant to reproduction are cholesterol and carotenoids.The former serves as a precursor to all steroid hormones, which regulate reproductive physiology [23].The latter play various roles, as antioxidants, precursors to vitamin A, and are involved in the protection of eggs against light [24], and have been shown to improve egg pigmentation, fertilization rate, and survival of rainbow trout when supplemented in the diet [25].
Because the various aspects of reproductive biolog y described above are still unknown for the shortfin corvina, the objective of the present investigation was to identify the gonadal developmental stages, proximate composition, fatty acid profile and other biochemical and biological indices of wild and cultured C. parvipinnis.

Wild fish
Wild, adult shortfin corvina with mean body weight (BW) of 1009.69 g for females and 835.93 g for males, were captured in November 2015, using an outboard-powered boat and seine nets at the estuary "Santa Rosa", latitude 28°58' N and longitude 112°10' W, near Kino Bay in the Gulf of California, Sonora, Mexico.Immediately after catching, approximately 1 mL of blood was drawn from the caudal vein of each fish with heparinized 1-cc syringes equipped with 25-gauge needles, and placed into 1.5-mL micro centrifuge tubes (Costar Corning, Incorporated, 1.5 mL Corning, New York, USA), and kept on ice.Subsequently, fish were killed by severing the spinal cord, placed into individually labeled bags, and kept on ice.All fish were transported to the Kino Bay Experiment Station (KBES), University of Sonora at Kino Bay, Sonora, Mexico, for further analysis.

Cultured fish
Juvenile shortfin corvina about 20 g of individual wet weight and 3 months of age after hatching, originating from wild broodstock acclimated to laboratory conditions and spawned in captivity, were obtained from the Center for Reproduction of Marine Species of the State of Sonora (CREMES), Kino Bay, Sonora, Mexico.Fish were transported and held at the KBES in a recirculating culture system consisting of two 10-m 3 fiberglass raceways (each with 8 m, length × 1.5 m, width × 1.0 m, height), in connection with a 3-m 3 sump tank, biofilter, a 1.5-HP pump (Jacuzzi, Model 150MF-T, Little Rock, Arkansas, USA), a sand filter (Jacuzzi, Model L-190-7, Little Rock, Arkansas, USA), a 120-Watt UV light chamber (Rainbow Lifeguard, Model UV97, El Monte, California, USA), and constant aeration supplied with a 1.0-HP blower (Fuji, Model VFC40, Saddle Brook, New Jersey, USA).Fish were fed a commercial extruded feed with crude protein and lipid contents of 46 and 12%, respectively (Marine MX, Skretting®, Tooele, Utah, USA), for 12 months.After this rearing period and at an approximate age of 15 months, all fish were anesthetized with tricaine methanesulfonate (MS-222, 300 mg L -1 ; Western Chemical, Inc., Ferndale, WA, USA) prior to blood collection and sacrifice, as previously described, and were then kept on ice for further analysis.The body indices and analyses subsequently described, including histology, proximate composition, fatty acid profile, etc., were performed on all fish unless indicated otherwise.

Fish dissection and tissue preparation
Fish were individually measured (total length (TL); cm) and weighed (BW; g) in order to calculate Fulton's condition factor (K)=(BW × 100)/TL 3 [26].The fish were then dissected to determine the weights of the viscera, gonads and liver to calculate other body indices, as follows: the viscerosomatic index (VSI) = (viscera weight, g/BW) x 100; the gonadosomatic index (GSI) = (gonad weight, g/BW) x 100, and the hepatosomatic index (HSI) = (liver weight, g/BW) x 100.Gonads from individual fish were partitioned into two similar portions.One portion was stored in Davidson's fixative solution (acetic acid:95% ethanol:formaldehyde:H 2 O, at a 1:3:2:3 ratio) for 24 h for histological determination of the gonad developmental stage.The other portion was stored at -84ºC and used for the determination of the fatty acid and proximate composition, cholesterol and total carotenoid contents.

Histological analyses of gonads
After fixation in Davidson's solution, the gonads were stored in a 70% ethanol solution for long-term storage, then immersed in ethanol solutions with progressively higher concentration (80, 95, 100%) for dehydration, and then in xylene for clearing.From each gonad, three segments about 1 cm in length, were sliced and embedded in paraffin.Transverse sections were cut, approximately 4 μm thick, placed onto microscope slides, stained with hematoxylin and eosin (H&E), and examined under a light microscope (Leica Microsystems, DM1000, Wetzlar, Germany) equipped with a digital camera (Moticam Pro, 282A), using the Motic Images Plus 2.0 software (Motic, Richmond, BC, Canada) for image acquisition.

Gonad proximate composition and fatty acid analysis
The proximate composition of gonads was determined in terms of crude protein, moisture, ash and crude fat.Using a Dumas Nitrogen Analyzer (Model NDA 702, VELP® Scientifica, Usmate, Italy), crude protein (N factor = 6.25) was analyzed via combustion by the Dumas method (method 968.06) [27].The contents of moisture and ash were determined by standard methods (methods 930.15 and 942.05, respectively) [27].Crude fat (CF) was extracted following a previously described method [28], and then quantified gravimetrically after drying a 5-mL aliquot under nitrogen.The remaining lipid extracted was employed for fatty acid analysis.The transesterification reaction of fatty acids was carried out using boron trifluoride.Fatty acid methyl esters (FAME) were analyzed with a Varian 3800 gas chromatograph equipped with a 30 m×0.25 mm fused silica capillary column and a flame ionization detector as described earlier [29].Fatty acids were identified by comparison of the retention times to those of known standards, quantified using heptadecanoic acid (17:0) as an internal standard, and expressed as mg g -1 of gonad wet weight.

Plasma 17β-estradiol (E2), gonad total carotenoid and cholesterol analyses
Blood samples were centrifuged (Heraeus Fresco 21, Thermo Scientific, Osterode am Harz, Germany) at 850×g for 15 min at 4°C to collect plasma, which was kept in micro centrifuge tubes at -20°C until E 2 analysis.Plasma E 2 concentration was determined by Enzyme-Linked Immune Sorbent Assay (ELISA), following a previously described procedure [30] and employing a commercial kit (MBS044518, My Biosource, San Diego, CA, USA).A fragment of the frozen gonad was used for the determination of total carotenoid and cholesterol content.Gonad tissue was immersed and homogenized in isotonic saline solution.Carotenoids were extracted from the crude homogenate using an acetone:methanol solution (2:1) and the concentration was determined by spectrophotometry, by measuring the absorbance at 495 nm [31].After centrifugation of the crude homogenate at 3000×g for 10 min, the supernatant was used for determination of the total cholesterol content by an ELISA method adapted for small samples [32].

Statistics
Descriptive statistics (means and standard error of the mean, SE) of fish length and weight, body indices, such as K, VSI, GSI, HSI, as well as of biochemical data, i.e. proximate composition and the contents of fatty acids, E 2 , total carotenoids and cholesterol, are presented.Excluding fish weight and length data, T-tests were conducted to compare wild and cultured organisms.Statistical analysis was conducted using the Statistical Analysis System (SAS 9.4) software package (SAS Institute Inc., [1999][2000].

RESULTS
Fifty wild shortfin corvinas (31 females and 19 males), in addition to 30 cultured fish (15 females and 15 males), were analyzed.Wild females, with mean BW and TL of 1009.69 g and 47.18 cm, respectively, were larger than their cultured counterparts, with mean BW of 448.39 g and TL of 36.43 cm.Wild males (BW of 835.93 g and TL of 44.07 cm) were also larger than cultured males (BW of 379.28 g and TL of 35.27 cm).Gender differences in size of fish of the same origin indicated that females were larger than males.The mean value of K was 0.96 for both wild females and males, whereas for cultured fish it was 0.92 for females and 0.86 for males.The differences were significant only when comparing wild vs. cultured males.The GSI values of wild fish (1.16 and 0.17% for females and males, respectively) were also statistically greater than those of cultured fish (0.39 and 0.05% for females and males, respectively), and were also greater in females as compared to males of the same origin.The mean values of VSI and HSI were calculated for wild fish only, with a numerically slightly higher VSI value in females (9.40%) vs. males (8.10%), while the means of HSI were relatively similar -1.95% in females and 2.05% in males (Table 1).
Since stages of gonad development have not yet been histologically classified for the shortfin corvina, histological samples were compared with those described for a related sciaenid species, the spotted seatrout, Cynoscion nebulosus [33].It was determined that gonads were in the early maturation stage for all wild (Fig. 1) and cultured (Fig. 2) male and female fish.For females, this stage is characterized by the presence of cortical alveolar oocytes, while some yolk granule vitellogenic oocytes also may be present.For males, continuous germinal epithelium can be seen throughout the testis, and all stages of spermatogenesis may be present, including spermatozoa in the lumen and ducts.
Although numerically similar, the crude protein content in gonads of wild males (14.83%) was statistically greater than in cultured males (14.00%), while the opposite was observed in cultured females (19.64%), as compared to wild fish (17.11%).The crude fat content also was statistically higher in the gonads of both cultured males and females (female: 10.85%, male: 20.48%), compared to wild fish (female: 2.05%; male: 11.05%), and it was also higher in males compared to females.Moisture was the major component of the gonads, ranging between 62.52 to 66.94% in cultured corvinas and between 70.08 to 75.56% in wild fish, and it was significantly higher in both male and female fish in comparison to their cultured counterparts.Ash was the smallest component, ranging from 1.09% in wild male gonads, to 2.27% in the gonads of cultured females (Table 2).
Cholesterol and carotenoids were determined only in the gonads of wild fish; in females, cholesterol was higher (10.52 mg g -1 ) than in males (3.26 mg g -1 ), but the carotenoids were slightly higher in males than in females (3.13 vs. 2.18 µg g -1 ).Plasma E 2 concentration in wild females was 0.007 ng mL -1 (SE = 0.001), while in cultured females it was 0.009 ng mL -1 (SE = 0.001).
In wild males, the mean value was 0.006 ng mL -1 (SE = 0.002), and for cultured male fish it was 0.007 ng mL -1 (SE = 0.001), without significant differences between groups.
As a general trend, it was observed that the fatty acid content was higher in the gonads of cultured fish than in their wild counterparts, with a few exceptions.For instance, in wild female gonads (Table 3), 18:0, 20:4n-6, 22:5n-6, 22:5n-3, 22:6n-3 and 24:4n-6 were more abundant, and 20:4n-6, 22:5n-6, 22:5n-3 and 22:6n-3 also were more abundant in the gonads of wild males (Table 4).In both sexes, PUFA and HUFA were the most abundant type of fatty acids, compared to saturated and monounsaturated fatty acids, although individually, 16:0 proved to be the most abundant fatty acid in the gonads of this species, ranging from 128.58 to 164.84 mg per gram of gonad wet weight.After palmitic acid, highly unsaturated fatty acids like DHA, ARA and EPA were the major fatty acids in gonadal tissue of wild and cultured females and males.Among these, the concentration of EPA was significantly greater in cultured fish, while DHA and ARA were statistically more abundant in wild fish.The concentrations of n-3 fatty acids (wild female: 137.03, cultured female: 140.85, wild male: 130.62 and cultured male: 165.35 mg g -1 of gonad wet weight) were quantitatively higher than that of the   n-6 fatty acids (wild female: 59.81, cultured female: 62.23, wild male: 46.84 and cultured male: 79.97 mg g -1 of gonad wet weight).The n-3/n-6 ratios were 2.34 and 2.27 for wild and cultured females, respectively, without significant differences between groups.For males, wild fish had significantly higher n-3/n-6 ratio (2.81 mg g -1 ) than cultured males (2.08 mg g -1 ) (Tables 3 and 4).

DISCUSSION
The results presented herein represent, to our knowledge, the first histological evaluation of gonad maturity and gonad biochemical characteristics for C. parvipinnis.All wild-caught fish, captured in November 2015 had gonads in the early maturation stage, which can be related to the time of year of fish collection.According to local fishermen's traditional knowledge, C. parvipinnis displays a seasonal reproductive migration, with large aggregations of fish starting to occur in the fishing area (estuary "Santa Rosa") in November, when shortfin corvina sport-fishing tournaments are held.Interestingly, C. othonopterus, a closely related sciaenid found in the same area of the Gulf of California, performs a well-documented reproductive migration to the Upper Gulf of California and the Colorado River Delta, starting from October through June, with peak spawning events in March and April [34].If migration of the shortfin corvina is like that of C. othonopterus, it is possible that all fish were sampled at the beginning of the migration event, when fish still need to actively feed in preparation for final gonad maturation and spawning.Therefore, they all had gonads in the    Total n-6: 18:2n-6, 18:3n-6, 20:3n-6, 20:4n-6, 22:5n-6, 24:4n early maturation stage.To elucidate not only the biochemical changes taking place during the process of gonad maturation throughout its reproductive migration, but also to obtain a complete classification of the gonad developmental stages, seasonal studies of the reproductive biology of the shortfin corvina are warranted.
Because gonad volume and weight increase as gamete development progresses, the GSI of fish is considered as an indicator of sexual maturity [35].In wild fish of the present study, GSI means recorded for females and males were 1.16 and 0.17%, respectively, which is consistent with the values reported for females and males (approximately 1.5 and 0.5%, respectively) of the sciaenid C. nebulosus captured at the beginning of the reproductive season, presumably also during the early maturation stage [36].For other members of the family Sciaenidae, GSI values vary with species and degree of gonad maturity.For example, for female and male Argyrosomus regius the range is approximately from 0.2 to 5.0% and from 0.05 to 0.70%, respectively, over a whole year [37].For wild Sciaenops ocellatus, values of 0.2-1.0% in wild females and of 0.02-0.40% in wild males have been reported [38].Over the May-October period, values ranging from approximately 0.8 to 5.0% for females and from 0.1 to 2.5% for males have been reported for Sciaena umbra [35].On the other hand, the concentration of plasma E 2 , which has been shown to decrease as gonad maturity progresses in fish, is considered as another indicator of gonad development [39].In the present study, levels found in wild females and males were 0.007 and 0.006 ng mL -1 , respectively.They are somewhat low, as compared to values reported for other sciaenids, e.g., wild A. regius, with values ranging from 0.0933 ng mL -1 in immature fish to 0.0210 ng mL -1 in ripe females, and from 0.0933 to 0.009 ng mL -1 in males [37].For the same species, but in a cultured fish stock, the concentration of plasma E 2 varied from 0.194 to 0.473 ng mL -1 from November through July [40].Once again, seasonal monitoring of the GSI and plasma E 2 in the shortfin corvina would provide a clearer insight into the variation of these parameters.
Histological examination of gonads showed that both cultured female and male fish also possessed gonads in the early maturation stage, as well as similar E 2 plasma concentrations (0.009 and 0.007 ng mL -1 for females and males, respectively), as compared to wild fish.However, their size was much smaller (female: 448.39 g cultured vs. 1009.69g wild; male: 379.28 g cultured vs. 835.93g wild) and so were their GSI values (female: 0.39% cultured vs. 1.16% wild; male: 0.05% cultured vs. 0.17% wild).One of the reasons the shortfin corvina has attracted interest as a candidate species for aquaculture is its manageable adult size.As compared to large-sized sciaenids, such as totoaba (Totoaba macdonaldi), this characteristic has positive implications, such as eliminating the need of highcost, large facilities for broodstock rearing/spawning, ease of broodstock handling, and presumably a shorter rearing time until reproductive age is attained.It is worthwhile pointing out that cultured fish were exclusively fed the high-quality compound feed described earlier.However, greater growth rates, and possibly shorter time to reach sexual maturity, may be expected by supplementing with fresh-frozen dietary components, such as squid, vitamin/mineral-enriched fish, shrimp, and krill, which are routinely used in broodstock diets of a variety of marine fish species, for example, Pagrus major and Hippoglossus hippoglossus [24,[41][42].
The indices VSI and HSI, calculated only in wild shortfin corvina, did not show apparent gender differences: VSI values of 9.40 and 8.10% for females and males, respectively, and HSI values of 1.95 and 2.05% for females and males, respectively.For wild red drum (S. ocellatus), HSI has been shown to peak in March-April (2.3 and 1.8% for females and males, respectively), when fish actively store lipids, then decline significantly in September-October (0.6% for both sexes) due to nutrient mobilization during reproduction [38].In recent comparative feeding trials, HSI of the shortfin corvina has varied from 0.78 to 1.45% [4][5].With respect to the VSI, comparable results (5.49-9.81%)were reported for cultured A. regius [43], but lower values, ranging from 2.53 to 2.78%, also were reported for cultured organisms of the same species [44].While HSI values of the present study are within the range of values previously reported, and VSI data in studies with different species show a large dispersion, the annual variation of the HSI and VSI for C. parvipinnis is yet to be determined.
For fish species in which increased energy reserves and ripe gonads account for a significant part of total body weight, especially in females, Fulton's condition factor can serve as an indirect indicator of gonad maturation.This phenomenon has traditionally been shown for various saltwater and freshwater fish species, such as, sardines, anchovies, and sturgeons, in which the highest K values generally coincide with seasonal peaks in reproductive activity [45][46][47].In the present study, although differences were detected between wild and cultured males, the variation of K values for both sexes was small, ranging from 0.86 to 0.96.These results are similar to values previously obtained for this species, 0.88-1.1 [4][5] and for other sciaenids, such as S. umbra (0.9-1.1), A. regius (0.99), and C. othonopterus (1.1-1.3)[35,44,48].Interestingly, significant changes in K were observed over the May-October period for both female and male S. umbra [35], an antecedent that prompts further investigation of seasonal changes in K for the shortfin corvina.
In the gonad proximate composition, the content of crude protein in wild females was slightly higher than in cultured fish (19.64 vs. 17.11%,respectively), but similar in males (14.83 and 14.00%, respectively).Since both groups of fish had gonads in the early maturation stage, this may explain the similarities.In addition, other studies have described similarities in the biochemical composition of tissues of wild and captive S. ocellatus [38].These values are within the range of results reported for other species, for example, 14.02 to 24.81% crude protein in gonads of female Epinephelus diacanthus at different stages of ovarian maturation [49].There were marked, significant differences in the crude fat content from wild and cultured shortfin corvina in the present study (female: 10.85% cultured vs. 2.05% wild; male: 20.48% cultured vs. 11.05%wild).This result may be attributed to apparent differences in the dietary supply of crude fat, unknown for wild organisms, while captive fish were fed solely a 12% CF compound feed.Large variability of the CF content in gonads at the same or different stages of development also has been observed in several species belonging to various families, including Scombridae, Serranidae and Siganidae, among others, which has been explained in terms of active mobilization of lipid reserves into gonads during vitellogenesis [49][50][51].Moisture content, with an overall range from 66.94 to 75.56% for wild and cultured fish in this study, accounted for much of the gonad proximate composition, while ash was the smallest component (1.09-2.27%).These results are consistent with values previously described, e.g.moisture and ash contents of 60.55-71.66%and 0.82-1.20%,respectively, in gonads of wild female E. diacanthus, and gonad moisture content of 72.0-79.9% in Ammodytes hexapterus, [49,52].The gonad carbohydrate contents observed in this study for wild and cultured females (3.19 and 0.31%, respectively) or for wild and cultured males (2.95 and 1.25%, respectively) are to some extent similar to the range of values (1.23-1.46%)determined for female E. diacanthus [49].
With respect to the gonad fatty acid composition, the overall FA content was numerically higher in cultured fish, which is likely a reflection of their higher lipid content.However, specific fatty acids, such as DHA and ARA, were found at significantly higher concentrations in wild females (DHA: 68.48 mg g -1 vs. 45.32 mg g -1 ; ARA: 39.33 mg g -1 vs. 7.30 mg g -1 ) and males (DHA: 57.30 mg g -1 vs. 45.83mg g -1 ; ARA: 27.67 mg g -1 vs. 8.80 mg g -1 ).This can be explained in terms of the relatively high abundance of HUFA in natural marine food webs [53], in conjunction with the direct effect dietary FAs have on the fatty acid profile of body tissues, as it has been demonstrated for a number of fishes [54][55][56], including the shortfin corvina [4].Furthermore, while saturated and monounsaturated fatty acids are preferentially catabolized as a source of energy, essential fatty acids such as DHA and EPA, can be selectively stored or deposited in tissue, such as gonads [10,57], which also helps explain the observed results.DHA and ARA, along with EPA, were quantitatively the main constituent HUFA in gonads of both wild and cultured C. parvipinnis.This pattern agrees well with the observations of many other marine fish species, in which high relative abundance of some or all of these FAs has been detected in the gonads [50,58].The important physiological role that these FAs play explains their presence in significant amounts in gonadal tissues.For example, EPA and ARA are precursors of thromboxanes, prostaglandins and leukotrienes [11], which not only participate in ovarian and testicular steroidogenesis, but also aid in the regulation of immune and inflammatory processes, hematological and cardiovascular activity, and renal and neural function [59].In addition, it has been suggested that ARA is involved in the development of the immune system of the fish embryo, as well as in the process of hatching and early larval performance [12].In turn, DHA is a major constituent of neural and visual tissues [60].
Cholesterol serves as a precursor to all steroid hormones, which regulate reproductive physiology [23].For wild E. diacanthus, it was reported that cholesterol levels in ovaries decreased from 2.06% in stage I of development, to 1.27% in stage III, suggesting conversion of cholesterol into steroid hormones as gonad maturation progressed [49].In the present study, the concentration of cholesterol was 10.52 mg g -1 for females and 3.26 mg g -1 for males.Due to incompatible units, comparison to previous data [49] is unfortunately not possible.However, our results are comparable to the range of values (approximately 4-15 mg g -1 ) reported for the gonads of female and male Etroplus suratensis [61].Finally, the values of the total carotenoid contents, determined only for wild fish (2.18 µg g -1 in females and 3.13 µg g -1 in males), were similar to the data determined for E. diacanthus, in which values increased with the stage of gonad maturation, from 1.42 µg g -1 to 7.98 µg g -1 [49].Carotenoids in fish gonads are thought to play various roles relevant to fish reproduction, such as protection of eggs against light, precursors to vitamin A, and antioxidants [24], and have been shown to improve egg pigmentation, fertilization rate, and survival of rainbow trout when supplemented in the diet [25].Clearly, more information is needed on the quantitative changes of gonad cholesterol and carotenoids over the reproductive cycle of the shortfin corvina.

CONCLUSIONS
Populations of C. parvipinnis, one comprised of wild fish captured in November near Kino Bay, Gulf of California, and one composed of cultured organisms, possessed ovaries and testes in the early maturation stage.The gonadosomatic index, the concentration of gonad total cholesterol and carotenoid, as well as of plasma 17β-estradiol, are reported for the first time for this species.The indices K, VSI and HSI did not show apparent gender differences.Moisture content, with an overall range between 66.94 to 75.56%, was the main component of the gonad proximate composition, while ash was the smallest component (1.09-2.27%).Crude protein and fat ranged from 14.00 to 19.64% and from 2.05 to 20.48%, respectively.In the gonad, the overall FA content was higher in cultured fish.However, specific FAs, such as DHA and ARA, were found at significantly higher concentrations in wild organisms, which along with EPA, were quantitatively the main constituent HUFA in gonads of both wild and cultured fish.In general, the biochemical composition of the gonads of wild and cultured organisms at an early maturation stage was quite similar, and these data may serve as indicators of dietary requirements for a broodstock diet, although they additionally prompt further investigations of seasonal changes in the reproductive biology of the shortfin corvina.

Fig. 2 .
Fig. 2. Testes from wild (A) and cultured (B) C. parvipinnis at an early maturation stage, characterized by well-differentiated ducts (DC) with lumens (L), and the presence of spermatogonia and other stages of spermatogenesis in ducts (SGD) and in lumens (SGL).Magnification: A -20x; B -40x.

Table 1 .
Biological parameters of wild and cultured C. parvipinnis females and males.

Table 2 .
Proximate composition, cholesterol and carotenoids content in gonads from wild and cultured C. parvipinnis females and males.

Table 3 .
Fatty acid profile (mg of fatty acid g -1 of gonad wet weight) of gonads from wild and cultured females of C. parvipinnis.
Values are means and standard error of the mean (SE) of all fish (Wild female: 31; Cultured female: 15).

Table 4 .
Fatty acid profile (mg of fatty acid g -1 of gonad wet weight) of gonads from wild and cultured males of C. parvipinnis.
Values are means and standard error of the mean (SE) of all fish (Wild male: 19; Cultured male: 15).