by Chung Sze-bun. / Thesis (M.Ph.)--Chinese University of Hong Kong, 1987. / Bibliography: leaves 211-242.

This study focused on obtaining sound quantitative data on the reproductive biology, length and age compositions and growth of the snapper Pagrus auratus in the waters off Carnarvon at ca 25oS and Perth at ca 32oS on the west coast of Australia and at ca 34oS on the south coast of Western Australia. Sampling thus encompassed both sub-tropical and temperate waters and the geographical range within which this species is abundant in Western Australia. The resultant data were used to explore the ways in which the biological characteristics of P. auratus differ with latitude and thus water temperature. An intensive sampling regime for eggs and spawning individuals of P. auratus was conducted in Cockburn Sound, a large marine embayment in the Perth region at ca 32oS. The resultant data were used to elucidate where and when spawning occurs in this large marine embayment and to determine more precisely the factors that influence the timing of spawning. The implications of the results presented in this thesis for the management of P. auratus, a species that has been subjected to very heavy fishing pressure in recent years, are discussed. The time and duration of spawning of P. auratus in the subtropical waters of Carnarvon differed markedly from those recorded for this sparid in the temperate and cooler waters of the Perth and the south coast regions. Spawning at Carnarvon occurred predominantly in the five months between late autumn (May) and mid spring (September), whereas it took place mainly in the three months between mid spring (October) and early summer (December) in the Perth region. Spawning of P. auratus on the south coast occurred predominantly in October and November in 2003 and 2004 and scarcely at all in 2005. Gonadal recrudescence was thus initiated when water temperatures were close to their maximum but declining in Carnarvon, and close to their minima and rising in the Perth and south coast regions, respectively. The prevalence of fully mature and spawning females in all three regions was greatest in those months when water temperatures lay between 19 and 21oC. Collation of the data in this thesis and those provided in the literature for other populations showed that the spawning period was related to latitude, occurring far earlier in sub-tropical than temperate waters. The females and males attained maturity at a far smaller total length (L50) in the Carnarvon region, i.e. 378 and 353 mm, respectively, than in the Perth region, 585 and 566 mm, respectively, and also the south coast region, i.e. 600 and 586 mm. The trends exhibited by the age at maturity parallel those for length, with the A50s for the two sexes increasing from ca 4 years in Carnarvon to ca 5.6 years in the Perth region and nearly 7 years in the south coast region. The L50 and A50 at maturity thus both increased with increasing latitude. Marginal increment analysis demonstrated that, irrespective of the number of opaque zones in the otoliths of P. auratus, a single such opaque zone is laid down each year in these otoliths. Furthermore, the trends exhibited by the monthly marginal increments showed that the opaque zone is laid down predominantly in winter in the subtropical waters of Carnarvon, as opposed to mainly in spring in the temperate waters of the Perth and south coast regions. Thus, the timing of formation of the opaque zone in the otoliths of P. auratus along the Western Australian coast is not related to the trends exhibited by water temperature, but, in both the Carnvarvon and Perth regions, was coincident with the timing of spawning. The maximum total lengths recorded for females and males in the Carnarvon region, i.e. 864 and 840 mm, respectively, were considerably less than the corresponding values of 1051 and 1056 mm in the Perth region, and 1083 and 1099 mm in the south coast region. Growth in the Perth and south coast regions was greater than in Carnarvon, as is reflected in, for example, the respective lengths of 820, 720 and 610 mm for females at 10 years of age, as determined from the von Bertalanffy growth equations. The length and age compositions in the Carnarvon and south coast regions were essentially unimodal, whereas those in the Perth region were bimodal. However, the “mode” in the length-frequency distribution for the south coast region was located well to the right of that in the Carnarvon region, reflecting relatively lower contributions by individuals of the age cohorts of 3 to 6 years. The marked bimodality in the length-frequency distribution for P. auratus in the Perth region was due to the presence of a group of mainly smaller individuals caught outside Cockburn Sound and another of mainly larger individuals that were caught in Cockburn Sound and which formed part of a spawning aggregation in that embayment. The proportion of fish > 10 years old in the Carnarvon region declined markedly between 2003 and the following two years, presumably reflecting the effect of heavy fishing pressure. This contributed to the decision by fisheries managers to reduced the TAC in those waters after 2003. Age-frequency data demonstrated that annual recruitment success in Cockburn Sound varied greatly, with the 1991, 1992 and 1996 year classes being particularly strong, and recognizing that the relative numbers of the first two year classes did decline progressively between 2002 and 2004. Annual recruitment was particularly variable in the south coast region, with the catches of the 1996 year class dominating the samples. The relative number of early stage P. auratus eggs in ichthyoplankton samples collected from Cockburn sound on each of four new moons during the spawning seasons of four consecutive years peaked in November in three of those years, i.e. 2001, 2003 and 2004, and in November/December in the remaining year, i.e. 2002. This showed that spawning in this embayment peaked during these months, at which times the mean sea surface temperatures ranged only from 19 to 20oC. The prevalence of spawning fraction females in sequential samples demonstrated that spawning peaks at the new and, to a lesser extent, full moons. This helps account for the strong positive correlation between spawning fraction and tidal regime, with spawning being greatest when the tidal range is greatest. Spawning times, back-calculated from the ages of the eggs collected during ichthyoplankton surveys in Cockburn Sound on each of 16 new moons within the spawning periods of four successive years, demonstrated that, in this embayment, P. auratus spawns at night and within the first three hours of the onset of the ebb tide. The distribution of egg concentrations on the 16 new moons showed that, each year, spawning occurred firstly in the north-eastern area of Cockburn Sound and then in the middle and finally north-western areas of this embayment. In the Perth region, the marine embayments of Cockburn and Warnbro Sound act as nursery areas for P. auratus during the first two years of life. The majority of 2 to 5 year old fish and a large proportion of the 6 year old fish occupy the marine waters outside the embayments. The remaining 6 year old and almost all of the older fish begin to move in September into particularly Cockburn Sound, where they form relatively large spawning aggregations between October and December, before undergoing a massive emigration from this embayment in December/January. The limited returns from fish that were tagged in Cockburn Sound and were subsequently caught outside this embayment indicate that, following spawning, P. auratus does not tend to move in a particular direction. Pagrus auratus stocks are heavily exploited in offshore, oceanic waters and in embayments, such as Cockburn Sound, where they are particularly susceptible to capture because of the tendency of this species to form spawning aggregations in these same easily accessible locations each year. The data obtained during this thesis show that the L50 at maturity of females and males in temperate waters, i.e. nearly 600 mm, is far greater than the current minimum legal length (MLL) of 410 mm TL. There is thus a need to increase the MLL and/or reduce fishing pressure on immature individuals in open waters. However, the effectiveness of an increase in the MLL may be limited because there is evidence that P. auratus suffers from fishing-induced barotrauma. Closures of specific areas during the spawning season of P. auratus, such as those that have been applied in Cockburn Sound and Shark Bay, are potentially a very effective method for reducing the effects of heavy fishing on spawning individuals.

Snapper, Pagrus auratus is a valuable coastal fish species in New Zealand and forms an important commercial and recreational fishing industry in the north-east of New Zealand. Previous studies revealed evidence that this carnivorous, primarily benthic feeder consumes a non-indigenous macrobenthic species. Many non-indigenous macrobenthic species have now become established in New Zealand waters. For example, in Rangitoto Channel, Hauraki Gulf, non-indigenous macrobenthic species are prolific, with three bivalve species in particular having thriving populations: Limaria orientalis, Musculista senhousia, and Theora lubrica. The role of these species in the diet of snapper, however, is unknown. To assess the availability of indigenous and non-indigenous prey species to snapper, benthic macrofaunal assemblages throughout Rangitoto Channel were surveyed. To do so, sediment samples were collected at 84 sites. At 24 of these sites sediment was also collected for grain size analysis and at 40 of these sites the seafloor was surveyed with video. To investigate the diet of snapper, fish were collected from four monitoring sites within the channel. Bimonthly monitoring of the diet of snapper as well as the benthic macrofauna was completed at these monitoring sites and trends in the abundance of three prey species, two of which were non-indigenous species, within the sediment and the diet of snapper were compared from June to December 2008. A detailed description of the benthic macrofaunal assemblages throughout Rangitoto Channel confirmed that three non-indigenous species are established throughout this area. The analyses revealed that the diet of snapper has shifted compared to previous studies. Snapper now consume large quantities of two non-indigenous species, M. senhousia and L. orientalis. Consumption of the former species apparently results from its dominance and biomass within the sediment. It is therefore not surprising that snapper consumed large amounts of this species. In contrast, L. orientalis occurred disproportionately in the diet of snapper compared to its abundance within the sediment. I suggest that the establishment of some non-indigenous species benefits snapper.

Snapper, Pagrus auratus is a valuable coastal fish species in New Zealand and forms an important commercial and recreational fishing industry in the north-east of New Zealand. Previous studies revealed evidence that this carnivorous, primarily benthic feeder consumes a non-indigenous macrobenthic species. Many non-indigenous macrobenthic species have now become established in New Zealand waters. For example, in Rangitoto Channel, Hauraki Gulf, non-indigenous macrobenthic species are prolific, with three bivalve species in particular having thriving populations: Limaria orientalis, Musculista senhousia, and Theora lubrica. The role of these species in the diet of snapper, however, is unknown. To assess the availability of indigenous and non-indigenous prey species to snapper, benthic macrofaunal assemblages throughout Rangitoto Channel were surveyed. To do so, sediment samples were collected at 84 sites. At 24 of these sites sediment was also collected for grain size analysis and at 40 of these sites the seafloor was surveyed with video. To investigate the diet of snapper, fish were collected from four monitoring sites within the channel. Bimonthly monitoring of the diet of snapper as well as the benthic macrofauna was completed at these monitoring sites and trends in the abundance of three prey species, two of which were non-indigenous species, within the sediment and the diet of snapper were compared from June to December 2008. A detailed description of the benthic macrofaunal assemblages throughout Rangitoto Channel confirmed that three non-indigenous species are established throughout this area. The analyses revealed that the diet of snapper has shifted compared to previous studies. Snapper now consume large quantities of two non-indigenous species, M. senhousia and L. orientalis. Consumption of the former species apparently results from its dominance and biomass within the sediment. It is therefore not surprising that snapper consumed large amounts of this species. In contrast, L. orientalis occurred disproportionately in the diet of snapper compared to its abundance within the sediment. I suggest that the establishment of some non-indigenous species benefits snapper.

This thesis describes research designed to increase our knowledge of the nutritional requirements of Australian snapper Pagrus auratus and provide information on the potential of Australian feed ingredients to reduce the level of fishmeal in diets for this species. The apparent digestibility of organic matter (OM), crude protein (CP), crude fat (CF) and gross energy (GE) from selected animal, cereal or oilseed meals incorporated at different inclusion levels was determined. Snapper were extremely efficient at digesting the CP, CF and GE from fishmeal and rendered animal meals (range 80-100%) with the exception of meat meal, where CP and GE digestibility were lower (62-65%). The CP from oilseeds was better digested (87-91%) than OM (57%) or GE (64-67%). Digestibility of nutrients and GE from animal meals and fish oil was not influenced by inclusion level. The CP from extruded wheat was highly digestible (100-105%), but, the OM, CF and GE digestibility of extruded wheat declined as inclusion levels increased. The interactive effects of inclusion level (150, 250, 350 or 450 g kg-1) and fish size (110 vs 375 g snapper) on the apparent digestibility of OM and GE from gelatinised wheat starch were investigated. The OM and GE digestibility of gelatinised wheat starch was high (89%) at low inclusion levels, but declined significantly in both fish sizes as the level of starch increased. There was no interaction between inclusion level and size of fish and the decline in GE digestibility could be predicted by the regression; GEADC = 104.97(±3.39) - 0.109(±0.010) x inclusion level (R2=0.86). Larger fish were more capable of digesting the GE from gelatinised starch than smaller fish. Regardless of fish size, short and longer-term changes in the physiology of snapper fed or injected with carbohydrates were recorded. Liver and tissue glycogen concentrations and the hepatosomatic index (HSI) of snapper fed gelatinised starch were significantly elevated. The plasma glucose concentrations of fish injected intra-peritoneally with D-glucose increased from resting levels (0.4-4.6 mM) to 18.9 mM approximately 3 hours after injection and fish displayed a hyperglycaemic response for nearly 18 hours. In contrast, the post-prandial response to the uptake of glucose from normally digested gelatinised starch was more regulated. A dose-response study to determine the effects of digestible energy (DE) content (15, 18 or 21 MJ kg-1) on the digestible protein (DP) requirements of juvenile snapper was assessed using a four parameter mathematical model for physiological responses (4-SKM). DP content of test diets ranged from 210 to 560 g kg-1. Weight gain and protein deposition was strongly dependent on the ratio of DP:DE. According to the fitted models, diets for snapper weighing between 30-90 g and reared at temperatures ranging from 20-25ºC should contain a minimum of 28 g DP MJ DE-1 to promote optimal weight gain and protein deposition. The effect of varying the absolute content of DP and DE on the weight gain and performance of snapper (100-300 g) fed diets formulated with an optimal ratio of DP:DE was investigated. In addition, non-protein sources of DE were varied by adjusting the ratio of fish oil to gelatinised wheat starch in order to determine if different ratios of these ingredients affected performance. High-energy diets (22-23 MJ DE kg-1) suppressed feed intake, but provided DP intake was not limited by feed intake, maximum weight gain was approached. Lower-energy, lower-protien diets (15-18 MJ DE & 315-390 DP) encouraged higher feed intake but DP intake was restricted, which reduced growth potential. Snapper performed best on high-energy, high-protein diets (490 DP & 21 MJ DE), provided a significant proportion of DE was supplied as DP. Fish oil and pregelatinised wheat starch could be interchanged according to their DE values without unduly affecting fish performance in diets providing 390-490 g DP kg-1. Two utilisation studies were undertaken to investigate the performance of snapper fed diets containing increasing levels of poultry offal meal, meat meal and soybean meal. All diets were formulated with similar DP and DE contents. Snapper readily accepted feeds containing high levels of poultry meal (360 g kg-1), meat meal (345 g kg-1) or soybean meal (420 g kg-1), before weight gain and performance was negatively affected. In combination, these feed ingredients were able to replace all but 160 g fishmeal kg-1 in commercially extruded test feeds for this species. The research described in this thesis has extended knowledge of the nutritional requirements of Australian snapper by providing important information on the digestibility of Australian feed ingredients. These coefficients have been integral in formulating both experimental and semi-commercial test diets for snapper and will increase both the accuracy and flexibility of commercial diet formulations for this species. High performance feeds for snapper will contain high levels of DP, but must provide a significant proportion of DE in the form of protein. These constraints can be satisfied by using alternative, well-digested protein and energy sources that have the potential to replace all but 160 g kg-1 fishmeal.

Worldwide, an increasing demand for fish and fisheries products, together with socioeconomic pressure for industry expansion, is placing considerable pressure on wild fish stocks – more than 80% of which are considered by the Food and Agriculture Organisation of the United Nations (FAO) to be either maximally- or over-exploited. Adding value to the existing catch and/or improving the sustainability of current wild capture methods may offer a means of providing industry growth while negating the need for increased landings. In particular, the peri-mortem condition of a fish plays an integral role in the condition of the tissues post-mortem and hence in product quality, with harvesting techniques that result in stress or fatigue yielding a lower quality product. An understanding of the physiology of the target species and its response to harvest is therefore essential to implementing targeted improvements in harvesting technologies. For species harvested using trawl-based technologies, this includes knowledge of their exercise physiology, in particular their swimming capacity, since this is a key determinant of the interaction between fish and trawl gears, and hence of the nature and severity of stress experienced and of the condition of fish at landing. This thesis describes a series of discrete studies relating to the exercise physiology of juvenile snapper, Pagrus auratus, an iconic New Zealand finfish that comprises important recreational and commercial fisheries. In particular, we sought to characterise the capacity of snapper for sustained swimming activity, including how performance may differ between fish of different size or with environmental temperature; to determine the consequences of exhaustive exercise for both subsequent swimming activity, an important determinant of survival in escaping or discarded catch, and for tissue biochemistry, which ultimately determines product quality in harvested fish; to validate the use of laboratory-based simulations for the study of capture-related stress by comparing the response of laboratory-exercised snapper with commercially caught fish; and to determine the tolerance of snapper to environmental hypoxia, and further, the possible consequences of hypoxia for swimming capacity and for recovery in fish retained for subsequent rested-type harvest. The capacity of snapper for sustained swimming activity was characterised through the use of incremental exercise tests to determine critical swimming speeds, Ucrit. Juvenile snapper (94-107 mm length, 16-157 g mass) demonstrated a strong swimming capacity, with individual fish attaining critical swimming speeds of up to 7.1 body lengths per second (bl s⁻¹). Swimming performance demonstrated an allometric association, with absolute critical speeds increasing with fish size, whilst relative performance favoured smaller fish. The relation was described by the function Ucrit (m s⁻¹) = 0.003412 [length (mm)] + 0.2669. Critical swimming performance also exhibited variation in response to environmental variables. Thermal performance curves were evident in snapper acclimated to 12, 18 and 24 °C, with the suggestion of optimal performance at acclimation temperatures between 18 and 24 °C. Critical swimming performance was also significantly reduced during exposure to ambient oxygen tensions below 80 mmHg; at 40 mmHg, snapper attained only 21% of the critical swimming speeds observed under normoxic (150 mmHg) conditions. In juvenile snapper (~75 g), exhaustive exercise resulted in severe metabolic, acid-base, haematological and hormonal perturbations, the nature of which were similar to those classically demonstrated in other strong-swimming fish species, especially salmonids. These included the depletion of glycogen from within the white muscle (WM) and the concomitant production of lactate, with a resultant lactacidosis of the plasma; recruitment of erythrocytes from the spleen; and the release of cortisol to the plasma. The recovery of these disturbances required 6 hours under laboratory conditions. As the stresses experienced by fish during commercial capture are often considered to be greater than those which can be induced during laboratory-based simulations, it was necessary to investigate whether the magnitude of the perturbations observed in laboratory-exercised snapper were an appropriate model of those of trawl-caught fish. In trawl-caught snapper (1100 g, 38 cm) obtained under commercially-relevant conditions (tow speed ~3.0 knots; duration 2.25-2.75 hours), the magnitude of the perturbations were greater than for laboratory-exercised fish. While the recovery of some metabolites was evident within the first 18 hours post-capture, their recovery was prolonged relative to laboratory-exercised fish; other metabolites, namely muscle glycogen and plasma cortisol, exhibited no signs of recovery. These observations suggest that the response of snapper to exhaustive exercise within the laboratory may underestimate the severity of the response induced by commercial harvest. This is further suggested by post-capture mortality rates of 14%, whereas no mortality was observed following fatigue at Ucrit. Exhaustive exercise also resulted in the impairment of subsequent critical swimming performance. Immediately following fatigue, snapper (85-160 g) were capable of sustained swimming activity at speeds of up to 60-70% Ucrit; however, critical swimming performance was reduced 30%, presumably due to limitations in WM function. There was no suggestion of the recovery of WM function within the first 30 minutes post-fatigue; thereafter, Ucrit was progressively restored, such that snapper were able to repeat their initial swimming performance in a second Ucrit test performed 2 hours after the conclusion of the first. Snapper were moderately tolerant of hypoxia, oxygen-regulating at reduced oxygen tensions (<100 mmHg) by virtue of increased ventilatory rate and stroke volume, with a distinct bradycardia developing at PO₂ below 60 mmHg. Larger snapper appeared to possess a greater hypoxia tolerance than did smaller fish, with Pcrit resolved to 77 in 20 g fish, and 50 mmHg in 150 and 230 g fish. Exposure to moderate hypoxia (60-80 mmHg) during recovery from an exhaustive exercise event constrained MO₂ max to 78% of that of normoxic fish, however did not appear to impede the return of MO₂ to routine levels. The present study is the first to examine in detail the swimming performance of snapper, and the consequences of exhaustive exercise for physiological condition. By understanding the swimming capacities of snapper, it may be possible to refine harvesting practices (i.e. tow speeds) or utilise technologies (i.e. net design) such that the water velocities through the trawl net are within the range at which the fish can swim sustainably, minimising the extent of stress and fatigue experienced by fish, and hence their effects on both quality and survival. The study also demonstrates that whilst snapper experience significant physiological disturbance during commercial harvesting, including significant mortality, some fish demonstrate the potential for metabolic recovery, which may permit their retention in an on-board tank facility for subsequent rested-type harvest. Finally, the present work highlights a number gaps in our understanding of the link between harvesting conditions and fish condition, and makes a number of suggestions for future studies or directions.