Performance of juvenile Murray cod, Maccullochella peelii peelii (Mitchell), fed with diets of different protein to energy ratio

The results of a 56-day experiment on juvenile Murray cod, Maccullochella peelii peelii, an Australian native fish with a high aquaculture potential, of mean weight 14.9 ± 0.04 g, fed with five experimental diets, one a series of 40% protein content and lipid levels of 10, 17 and 24% (P40L10, P40L17 and P40L24), and another of 50% protein and 17 and 24% (P50L17 and P50L24) lipid are presented. The specific growth rate (SGR) (% day−1) of fish maintained on different diets ranged from 1.18 to 1.41, and was not significantly different between dietary treatments, except P40L10 and the rest. However, there was a general tendency for SGR to increase with increasing dietary lipid content at both protein levels. The food conversion ratio (FCR) for the 40% protein series diets were poorer compared with those of the 50% protein diets, and the best FCR of 1.14 was observed with the P50L17 diet. The protein efficiency ratio (PER), however, was better in fish reared on low protein diets. The net protein utilization (NPU) also did not differ significantly (P > 0.05) in relation to dietary treatment. As in the case of PER the highest NPU was observed in Murray cod reared on diet P40L24 and the lowest in fish fed with diet P50L24. The carcass lipid content reflected that of the diets, when significant increases in the lipid content was observed in relation to dietary lipid content at both protein levels. However, body muscle lipid content did not increase with increasing dietary lipid content, and was significantly lower than in the whole body. The fatty acids found in highest concentration amongst the saturates, monoenes and polyunsaturates (PUFAs) were 16 : 0, 18 : 1n-9 and 22 : 6n-3, respectively, and each of these accounted for more than 60% of each of the group's total. The muscle fatty acid content was affected by the dietary lipid content; for example the total amount (in μg mg−1 lipid) of monoenes ranged from 72 ± 5.1 (P40L10) to 112 ± 10 (P40L24) and 112 ± 2.8 (P50L17) to 132 ± 11.8 (P50L24) and the n-6 series fatty acids increased with increasing dietary lipid content, although not always significant. Most notably, 18 : 2n-6 increased with the dietary lipid level in both series of diets.

Growth and nutrient utilization of Murray cod Maccullochella peelii peelii (Mitchell) fingerlings fed diets with varying levels of soybean meal and blood meal

The Australian native freshwater fish Murray cod, Maccullochella peelii pellii (Mitchell), currently supports a fledgling inland aquaculture industry, which is thought to have considerable growth potential. The aim of this study was to evaluate the suitability of two alternate protein sources [blood meal (BM) and defatted soybean meal (SBM)] as substitutes for fish meal at various levels of inclusion in diets for juvenile Murray cod. The growth performance of juvenile Murray cod in response to nine isonitrogenous and isocalorific diets (50% protein, 14% lipid, 20.2 kJ g⁻¹) consisting of a control diet in which protein was supplied from fish meal, and test diets in which the fish meal protein was substituted at levels of 8%, 16%, 24%, and 32% with BM or SBM was evaluated from a 70-day growth experiment. The per cent apparent dry matter (% ADC_dm) and percentage protein digestibility (% ADC_p) of the test diets were also determined using Cr₂O₃ as a marker. Survival in all the SBM dietary treatments was high but that of fish on the BM dietary treatments was significantly (P < 0.05) lower than in all the other dietary treatments. Specific growth rate (% day−1) of Murray cod fed SBM incorporated diets ranged from 1.63 ± 0.06 to 1.78 ± 0.10 and even at the highest level tested (32% of the dietary protein from SBM) was not significantly different (P > 0.05) from the fish fed the control diet (1.65 ± 0.09). Feed conversion ratios of the SBM dietary treatments ranged from 1.36 ± 0.08 to 1.45 ± 0.07. The protein efficiency ratios and protein conversion efficiencies of Murray cod in the soybean meal treatments were also good and for a majority of the SBM diets were better than those for the control diet. Per cent ADC_dm and ADC_p of the SBM diets tested ranged from 70.6 ± 1.46 to 72.3 ± 1.81% and 88.6 ± 0.57 to 90.3 ± 0.17%, respectively, and was not significantly different (P > 0.05) from the control diet (% ADC_dm 74.3 ± 1.63; % ADC_p 91.3 ± 0.55). The reasons for significantly poor survival and growth of Murray cod reared on BM incorporated diets, and relatively poor digestibility of these diets are discussed. The study shows that for Murray cod diets in which fish meal protein is substituted up to 32% performance or carcass composition is not compromised.

Microsatellite loci for studies of wild and hatchery Australian Murray cod Maccullochella peelii peelii (Percichthyidae)

Murray cod is a species of conservation concern that is subject both to wild harvest and to aquaculture production. Six polymorphic microsatellite loci have been developed for this species, which will facilitate studies of wild-stock structure, will help the management of hatchery diversity, and will aid genetic improvement programmes. The markers exhibited nonindependence of genotypes between loci and deviations from Hardy–Weinberg expectations, although these most probably reflect practices at the hatchery from which the genotyped individuals were sourced.

Performance of intensively farmed Murray cod Maccullochella peelii peelii (Mitchell) fed newly formulated vs. currently used commercial diets, and a comparison of fillet composition of farmed and wild fish

Murray cod is a top-order carnivore with high culture potential. Currently, there are no commercial diets formulated specifically for Murray cod. In this study, results of two growth trials on Murray cod (80–83.5-g mean initial weight), conducted in commercial settings, using two laboratory-formulated diets (DU1 and DU2; 48.9% and 49.1% protein, and 16.9% and 16.1% lipid, respectively, on a dry matter basis), and two commercial diets, formulated for other species (salmon – CD/S and barramundi – CD/B) but used in Murray cod farming are presented. The two commercial diets had less protein (46.6% and 44.4%) but higher lipid (21.7% and 19.5%). The energy content of the feeds tested was similar (about 20–22 kJ g−1). The growth performance and feed utilization of Murray cod did not differ significantly amongst the diets, but the food conversion ratio and % protein efficiency ratio in fish fed the DU1 and DU2 diets were consistently better. There was significantly less carcass and muscle lipid deposition in fish fed with the latter diets. Of the fatty acids in muscle, the lowest amounts (in μg mg lipid−1) of n-3 (262.5±2.9), n-6 (39.8±0.9) and polyunsaturated fatty acid (PUFA) (302.3±3.8) were observed in fish fed CD/S, and the highest in fish fed DU2 and CD/B. Fatty acids 16:0 and 18:0, 18:1n-9 and 16:1n-7, and 22:6n-3, 20:5n-3, 22:5n-3 and 18:2n-6 were the dominant fatty acids amongst the saturates, monoenes and PUFA, respectively, and accounted for 80.8–88.7% of all identified fatty acids (23) in muscle of Murray cod. The study showed that Murray cod could be cultured successfully on a diet (DU2) containing 20% soybean meal without compromising growth and/or carcass quality. Differences in the proximate composition and fatty acid composition of muscle of wild and farmed Murray cod were observed, the most obvious being in the latter. Wild Murray cod had significantly less (P<0.05) saturates (192.6±1.84 vs. 266.3±3.51), monoenes (156.5±8.7 vs. 207.6±6.19), n-3 (145.2±5.24 vs. 261.8±3.2) but higher n-6 (144.3±2.73 vs. 48.3±1.38) in muscle (all values are in μg mg lipid−1) than in farmed fish. Wild fish also had a much lower n-3 to n-6 ratio (1.0±0.03 vs. 5.4±0.09).

Performance of murray cod, Maccullochella peelii peelii (Mitchell) in response to different feeding schedules

The Australian freshwater fish Murray cod, Maccullochella peelii peelii (Mitchell) is gaining popularity as a suitable species for intensive culture, particularly in closed systems. The aim of this study was to evaluate the performance of Murray cod in response to different feeding schedules. Growth, survival, food conversion and a range of other related parameters including carcass proximate composition were evaluated for fish in five feed management regimes. The feeding regimes used in the experiment were hand fed to satiation twice daily (SAT), a pre-determined ration of 1.2% of the body weight day−1 which was hand fed twice daily (HFR), and belt fed through the day only (B/D), belt fed through the night only (B/N) and belt fed for 24 h (B/DN). Each of the five feeding regimes was randomly allocated to three tanks (triplicates). All of the feeding regimes used a commercially prepared diet formulated specifically for Murray cod, containing ≈50% protein and ≈16% lipid. The experiment was conducted for 84 days. Specific growth rate ranged from 0.89±0.01 to 1.07±0.04% day−1. Food conversion ratio (FCR) ranged from 1.09±0.02 to 0.92±0.03. The fastest growth and greatest final body weight were observed in the SAT treatment; however, the highest FCR, visceral fat index (VFI %) and hepatosomatic index (HSI %) were also observed in this treatment. Significant differences were found in specific growth rate and final mean weight between fish in the B/D and SAT treatments. B/N and B/DN feeding regimes appeared to result in the most favourable fish performance.

Fatty acid metabolism in the freshwater fish Murray cod (Maccullochella peelii peelii) deduced by the whole-body fatty acid balance method

The whole-body fatty acid balance method was used to investigate the fatty acid metabolism in Murray cod (Maccullochella peelii peelii) fed diets containing canola (CO) or linseed oil (LO). Murray cod were able to elongate and desaturate both 18 : 2n − 6 and 18 : 3n − 3. In fish fed the CO diet, 54.4% of the 18 : 2n − 6 consumed was accumulated, 38.5% oxidized and 6.4% elongated and desaturated to higher homologs. Fish fed the LO diet accumulated 52.9%, oxidized 37% and elongated and desaturated 8.6% of the consumed 18 : 3n − 3. The overall roles of n − 6 fatty acids appeared more important in Murray cod compared to other freshwater species. Murray cod also showed a preferential order of utilization of C18 fatty acid for energy production (18 : 3n − 3 > 18 : 2n − 6 > 18 : 1n − 9). Moreover, it is demonstrated that an increase in dietary 18 : 3n − 3 is directly responsible of increased desaturase activity and augmented saturated fatty acid accumulation in the fish body. The present study also suggests that, in the context of the possible maximization of the natural ability of fish to produce long chain polyunsaturated fatty acids, the whole-body approach can be considered well suited and informative and Murray cod is a suited candidate to fish oil replacement for its diets.

Effects of dietary oil source on growth and fillet fatty Acid composition of Murray cod, Maccullochella peelii peelii

The Murray cod, an Australian native freshwater fish, supports a relatively small but increasing aquaculture industry in Australia. Presently, there are no dedicated commercial diets available for Murray cod; instead, nutritionally sub-standard feeds formulated for other species are commonly used. The aim of the present investigation was to assess the suitability of two plant based lipid sources, canola oil (CO) and linseed oil (LO), as alternatives to fish oil for juvenile Murray cod. Five iso-nitrogenous, iso-calorific, iso-lipidic semi-purified experimental diets were formulated with 17% lipid originating from 100% cod liver oil (FO), 100% canola oil, 100% linseed oil and 1 : 1 blends of canola and cod liver oil (CFO) and 1 : 1 blends of linseed and cod liver oil (LFO). Each of the diets was fed to apparent satiation twice daily to triplicate groups of 50 Murray cod with initial mean weights of 6.45 ± 1.59 g for 84 days at 22 °C. Final mean weight, specific growth rate and daily feed consumption were significantly higher for the FO and LFO treatments compared to the LO treatment. Feed conversion and protein efficiency ratios were not significantly different amongst treatments. Experimental diets containing vegetable oil and vegetable oil blend(s) had significantly higher concentrations of n-6 fatty acids, predominantly in the form of linoleic acid (LA), while n-3 fatty acids were present in significantly higher concentrations in LO and LFO treatments. The fatty acid composition of Murray cod fillet was reflective of the dietary lipid source. Fillet of fish fed the FO was highest in EPA (20:5n-3), ArA (20:4n-6) and DHA (22:6n-3). Fish fed the CO diet had high concentrations of oleic acid (OlA) (192.2 ± 10.5 mg g lipid− 1), while the fillet of Murray cod fed the LO diet was high in α-linolenic acid (LnA) (107.1 ± 6.7 mg g lipid− 1). The present study suggests that fish oil can be replaced by up to 100% with canola oil and by up to 50% with linseed oil in Murray cod diets with no significant effect on growth.

Modification of tissue fatty acid composition in Murray cod (Maccullochella peelii peelii, Mitchell) resulting from a shift from vegetable oil diets to a fish oil diet.

The dynamics of fatty acid composition modifications were examined in tissues of Murray cod fed diets containing fish oil (FO), canola oil (CO) and linseed oil (LO) for a 25-week period and subsequently transferred to a FO (finishing/wash-out) diet for a further 16 weeks. At the commencement of the wash-out period, following 25 weeks of vegetable oil substitution diets, the fatty acid compositions of Murray cod fillets were reflective of the respective diets. After transfer to the FO diet, differences decreased in quantity and in numerousness, resulting in a revert to the FO fatty acid composition. Changes in percentages of the fatty acids and total accumulation in the fillet could be described by exponential equations and demonstrated that major modifications occurred in the first days of the finishing period. A dilution model was tested to predict fatty acid composition. In spite of a general reliability of the model (Y=0.9234X+0.4260, R²=0.957, P<0.001, where X is the predicted percentage of fatty acid; Y the observed percentage of fatty acid), in some instances the regression comparing observed and predicted values was markedly different from the line of equity, indicating that the rate of change was higher than predicted (i.e. Y=0.4205X+1.191, R²=0.974, P<0.001, where X is the predicted percentage of α-linolenic acid; Y the observed percentage of α-linolenic acid). Ultimately, using the coefficient of distance (D), it was shown that the fatty acid composition of fish previously fed the vegetable oil diets returned to the average variability of the fillet fatty acid composition of Murray cod after 70 or 97 days (LO and CO respectively).

Effect of crude oil extracts from trout offal as a replacement for fish oil in the diets of the Australian native fish Murray cod Maccullochella peelii peelii

The efficacy of trout oil (TO), extracted from trout offal from the aquaculture industry, was evaluated in juvenile Murray cod Maccullochella peelii peelii (25.4-0.81 g) diets in an experiment conducted over 60 days at 23.7-0.8 °C. Five isonitrogenous (48% protein), isolipidic (16%) and isoenergetic (21.8 kJ gm1) diets, in which the fish oil fraction was replaced in increments of 25% (0-100%), were used. The best growth and feed efficiency was observed in fish fed diets containing 50-75% TO. The relationship of specific growth rate (SGR), food conversion ratio (FCR) and protein efficiency ratio (PER) to the amount of TO in the diets was described in each case by second-order polynomial equations (P<0.05), which were: SGR=-0.44TO2+0.52TO+1.23 (r2=0.90, P<0.05); FCR=0.53TO2-0.64TO+1.21 (r2=0.95, P<0.05); and PER=-0.73TO2+0.90TO+1.54 (r2=0.90, P<0.05). Significant differences in carcass and muscle proximate compositions were noted among the different dietary treatments. Less lipid was found in muscle than in carcass. The fatty acids found in highest amounts in Murray cod, irrespective of the dietary treatment, were palmitic acid (16:0), oleic acid (18:1n-9), linoleic acid (18:2n-6) and eicosapentaenoic acid (20:5n-3). The fatty acid composition of the muscle reflected that of the diets. Both the n-6 fatty acid content and the n-3 to n-6 ratio were significantly (P<0.05) related to growth parameters, the relationships being as follows. Percentage of n-6 in diet (X) to SGR and FCR: SGR=-0.12X2+3.96X-32.51 (r2=0.96) and FCR=0.13X2-4.47X+39.39 (r2=0.98); and n-3:n-6 ratio (Z) to SGR, FCR, PER: SGR=-2.02Z2+5.01Z-1.74 (r2=0.88), FCR=2.31Z2-5.70Z+4.54 (r2=0.93) and PER=-3.12Z2-7.56Z+2.80 (r2=0.88) respectively. It is evident from this study that TO could be used effectively in Murray cod diets, and that an n-3:n-6 ratio of 1.2 results in the best growth performance in Murray cod.

Dietary lipid source modulates in vivo fatty acid metabolism in the freshwater fish, Murray cod (Maccullochella peelii peelii)

The aim of the present investigation was to quantify the fate of C18 and long chain polyunsaturated dietary fatty acids in the freshwater fish, Murray cod, using the in vivo, whole-body fatty acid balance method. Juvenile Murray cod were fed one of five iso-nitrogenous, iso-energetic, semipurified experimental diets in which the dietary fish oil (FO) was replaced (0, 25, 50, 75, and 100%) with a blended vegetable oil (VO), specifically formulated to match the major fatty acid classes [saturated fatty acids, monounsaturated fatty acids, n-3 polyunsaturated fatty acids (PUFA), and n-6 PUFA] of cod liver oil (FO). However, the PUFA fraction of the VO was dominated by C18 fatty acids, while C20/22 fatty acids were prevalent in the FO PUFA fraction. Generally, there was a clear reflection of the dietary fatty acid composition across each of the five treatments in the carcass, fillet, and liver. Lipid metabolism was affected by the modification of the dietary lipid source. The desaturation and elongation of C18 PUFAs increased with vegetable oil substitution, supported by the occurrence of longer and higher desaturated homologous fatty acids. However, increased elongase and desaturase activity is unlikely to fulfill the gap observed in fatty acid composition resulting from decreased highly unsaturated fatty acids intake.

Effects of fish oil substitution with a mix blend vegetable oil on nutrient digestibility in Murray cod, Maccullochella peelii peelii

The replacement of fish oil with vegetable based oils is a strategy which is increasingly being adopted by the aquafeed industry as an essential component to reduce the reliance on a limited supply of a natural resource, which in turn also provides cost benefits. The aim of the present investigation was to evaluate the lipid and fatty acid digestibility of a mix blend vegetable oil in juvenile Murray cod; an emerging species of increasing interest in the Australian aquaculture sector. Five semi purified experimental diets were formulated with 17% lipid originating from fish oil (FO) which was substituted in 25% increments by blended vegetable oil (VO), formulated using olive oil (12%), palm oil (43%) and linseed oil (45%) to match the major fatty acid classes of the FO.

Significant differences between the dietary treatments with regard to the lipid and individual fatty acid digestibility were recorded. This study clearly demonstrated that in Murray cod fatty acid digestibility decreases with chain length, and inversely increases relative to the degree of unsaturation. The combination of chain length, degree of unsaturation and the melting point of individual fatty acids resulted in the digestibility of PUFA > MUFA > SFA and short chain > longer chain fatty acids. Therefore, the inclusion of a mixed blend vegetable oil resulted in significantly reduced lipid digestibility in juvenile Murray cod.

Lipid characterisation and distribution in the fillet of the farmed Australian native fish, Murray cod (Maccullochella peelii peelii)

The objective of this study was to determine the distribution pattern of lipids and fatty acids in different tissues of farmed Murray cod (Maccullochella peelii peelii).

Differences in lipid content were found amongst different portions of the fillet, being lowest in the dorsal/cranial portion (P1) and highest in the more ventral/caudal portion (P8) (P < 0.05). The latter also recorded the highest amount of monounsaturated fatty acid (MUFA) and the lowest in polyunsaturated fatty acids (PUFA), arachidonic acid, 20:4n − 6 (ArA), docosahexaenoic acid, 22:6n − 3, (DHA) and the n3/n6 ratio. In general, lipid content in the different fillet portions was inversely correlated to PUFA and directly to MUFA. Contents of saturated fatty acids (SFA) and eicosapentaenoic acid, 20:5n − 3 (EPA) did not show any discernible trends in the different fillet portions, while significant differences in contents of DHA and ArA were observed. This study shows that lipid deposition in Murray cod varies markedly and that different fatty acids are deposited differently throughout the fillet.

Growth performance, feed efficiency and fatty acid composition of juvenile Murray cod, Maccullochella peelii peelii, fed graded levels of canola and linseed oil

n two independent experiments, the effects of dietary inclusion of canola and linseed oil were evaluated in juvenile Murray cod (Maccullochella peelii peelii, Mitchell) over a 112-day period. In each experiment, fish received one of five semi-purified diets in which the dietary fish oil was replaced with canola oil (Experiment A) or linseed oil (Experiment B) in graded increments of 25% (0–100%). Murray cod receiving the graded canola and linseed oil diets ranged in final weight from 112.7 ± 7.6 to 73.8 ± 9.9 g and 93.9 ± 3.6 to 74.6 ± 2.2 g, respectively, and exhibited a negative trend in growth as the inclusion level increased. The fatty acid composition of the fillet and liver were modified extensively to reflect the fatty acid composition of the respective diets. Levels of oleic acid (18:1 n-9) and linoleic acid (18:2 n-6) increased with each level of canola oil inclusion while levels of α-linolenic acid (18:3 n-3) increased with each level of linseed oil inclusion. The concentration of n-3 highly unsaturated fatty acids in the fillet and liver decreased as the amount of vegetable oil in the diets increased. It is shown that the replacement of fish oil with vegetable oils in low fish meal diets for Murray cod is possible to a limited extent. Moreover, this study reaffirms the suggestion for the need to conduct ingredient substitution studies for longer periods and where possible to base the conclusions on regression analysis in addition to anova.

Finishing diets stimulate compensatory growth: results of a study on Murray cod, Maccullochella peelii peelii

The effective implementation of a finishing strategy (wash-out) following a grow-out phase on a vegetable oil-based diet requires a period of several weeks. However, fish performance during this final stage has received little attention. As such, in the present study the growth performance during both, the initial grow-out and the final wash-out phases, were evaluated in Murray cod (Maccullochella peelii peelii). Prior to finishing on a fish oil-based diet, fish were fed one of three diets that differed in the lipid source: fish oil, a low polyunsaturated fatty acid (PUFA) vegetable oil mix, and a high PUFA vegetable oil mix. At the end of the grow-out period the fatty acid composition of Murray cod fillets were reflective of the respective diets; whilst, during the finishing period, those differences decreased in degree and occurrence. The restoration of original fatty acid make up was more rapid in fish previously fed with the low PUFA vegetable oil diet. During the final wash-out period, fish previously fed the vegetable oil-based diets grew significantly (P < 0.05) faster (1.45 ± 0.03 and 1.43 ± 0.05, specific growth rate, % day−1) than fish continuously fed with the fish oil-based diet (1.24 ± 0.04). This study suggests that the depauperated levels of highly unsaturated fatty acids in fish previously fed vegetable oil-based diets can positively stimulate lipid metabolism and general fish metabolism, consequently promoting a growth enhancement in fish when reverted to a fish oil-based diet. This effect could be termed 'lipo-compensatory growth'.

Evaluation of weaning strategies for intensively reared Australian fish, Murray cod, Maccullochella peelii peelii

The Australian Murray cod supports a growing national industry. However, with regard to the process of weaning fry, there is a lack of information and optimal procedures need to be developed. The aim of the present investigation was to test the biological and economic efficacy of different weaning strategies for Murray cod. Three weaning strategies were tested on triplicate groups of fish: (1) only Artemia for 5 d, 7 d on Artemia plus starter diet, and 14 d on dry diet only; (2) 12 d on Artemia plus starter diet and 14 d on dry diet only; and (3) directly to dry diet for the entire experimental period. No significant differences were recorded in the growth and feed efficiency, while significantly higher mortality (38.4 ± 0.35%) was recorded in fish weaned directly onto dry diet. Fish subjected to the first 5 d on Artemia only showed a growth reduction during this period, which was compensated by a phase of enhanced growth during the dry-diet phase. No significant differences were noted in the proximate composition of fish under the different treatments. The economic evaluation suggested that the treatment with the simultaneous supply of Artemia and starter diet is preferable.