Unlocking the secrets of bream sex

Dr James Haddy

Dr James Haddy is a lecturer for the Australian Maritime College at Beauty Point. Over the past 10 yrs he has actively researched a variety of aspects of the biology and physiology of black bream. Below is a brief summary of some of his findings on reproduction in black bream.

An old timer "Merv" at Swansea once told me that he goes bream fishing once the cherry trees had flowered. Well it's that time of year again, the stone fruit trees have flowered and many of you are dusting off your bream rod.  What have flowering fruit trees got in common with bream fishing I hear you ask?  Well actually a lot!  You see, after the winter solstice on June 21 the days start to get longer. The fruit trees respond to this change in day-length by initiating their reproductive systems to produce flower buds. Similarly, black bream also respond to this environment cue as it signals that spring is on it way. Bream detect the change in day-length by using a small photosensitive gland called the pineal gland.  The pineal gland is located on top of the frontal brain lobes where the skull is at its thinnest. As the days start to get longer the pineal gland is sensitive to the longer days, due to more light, and secretes less melatonin. In response to these decreasing melatonin levels a suite of hormonal changes occur that prepare the fish for the up and coming spawning season by stimulating the fish to produce eggs and sperm. This is a complex endocrine cascade that stimulates various physiological responses from the brain, pituitary, gonads and liver. Day-length however is not the only environmental cue that regulates reproductive activity in bream with temperature, dissolved oxygen levels and salinity also playing a part in co-ordinating their reproductive activity.  As a result, different estuaries across Tasmania display marked differences in the timing of reproductive activity. As a general rule the small shallow coastal estuaries with small water catchments will heat up quicker than larger deeper systems that have rivers flowing into them sourced from cold highland rain and snow melt. As a consequence these smaller systems tend to fish better earlier in the season than the larger estuaries as the fish increase their reproductive, feeding and schooling activities sooner.
During winter bream are dispersed throughout the estuaries and their gonads are very small weighing less than 2% of their total weight. However, once the bream detect that the seasons are changing the females start to produce yolk for their eggs and males start to put energy into developing their testis. During this time bream start to school up and begin feeding regularly to supply the increasing energy demands of their developing gonads. After forming into "small schools" they swim up the estuary into the upper estuarine reaches. This is usually a riverine section of the creek or river flowing into the estuary. As more bream arrive on the spawning grounds they form into a densely aggregated school. Recent research in NSW indicates that in some estuaries the density of bream can reach values as high as 1 individual per square meter over a 2 kilometre stretch of river. At this time the gonads of a bream have grown significantly and can contribute up to 20% of the total weight of the individual. Many science studies use the seasonal variation in the gonads size to define the spawning season of a fish by calculating the gonad to body weight index. This is known as the gonadosomatic index. Figure 1 displays the seasonal variation of this index and highlights that reproductive activity is highest between September to December in Tasmanian bream. Another way scientists assess fish reproduction is by visual examination of the gonads.  In winter the ovaries are small and a clear orange colour. This is an inactive or immature ovary (see figure 2). Eight weeks after the winter solstice the eggs start accumulating yolk and the ovaries become granular in appearance. This stage many of you would call "in roe" but scientifically we call it vitellogenic due to the accumulation of the yolk protein vitellogenin. Once the eggs have sufficient yolk and the environmental conditions are right, a proportion of eggs undergo a final maturation stage. During this stage the egg takes up water and the yolk and oil droplets coalesce. This stage is known as hydrated, but some of you may call this "ripe". Just before dusk these eggs rupture out of their follicles and become loose individual eggs in the ovaries cavity. This stage is known as ovulated, or "running ripe". This stage is the spawning stage and only last for about 1 hour at dusk. This final maturation process and ovulation can be switched on and off depending on the environmental condition the day before. This way the bream ensure that eggs are not spawned when conditions are inappropriate such when a spring flood occurs.
Black bream are a unique fish species as their entire life cycle can be completed within the estuary they live in. As a result many intermittently opening coastal lagoons also support good black bream populations around southern Australia. However all prime bream estuaries possess a good section of riverine environment. The riverine environment is an excellent place to spawn for bream as small zooplankton is abundant, which will be a rich food source for their babies (larvae). However, sometimes the bream arrive on the spawning grounds too soon. So they must wait until the environmental conditions become suitable for spawning. Typically bream spawn in spring and early summer when the water temperature is between 14-26oC, the dissolved oxygen is above 50% saturation (5 ppm) and when the sub-surface salinity is above 11 ppt. However there is an increasing suspicion that female bream also require a visual cue of zooplankton density prior to initiating spawning activity. This suspicion is supported by evidence that bream rarely spawn in an open estuarine environment where suitable water quality parameters are also present. In addition, egg density surveys find the majority of eggs in the upper riverine environment where zooplankton is also abundant (see figure 3). Sometimes, due to excessive rain the spawning sites are flooded by a flush of cold fresh water. During these events the bream cease all reproductive activities and their reproductive hormones disappear from the blood. If the flood is severe enough, they will retreat to the lower reaches of the estuary. Once the flood has abated the bream will return to the upper riverine environment to resume spawning. This is evidence that bream are a stress sensitive species.  Furthermore studies in Tasmania have shown that when bream are placed in a confinement tank, such as live well, the stress event causes the complete shutdown of their reproductive physiology within 1hr.  However whether these fish recover from this stress and/or the speed at which they resume spawning activities after being stressed is currently unknown.
Bream spawn on a daily basis at dusk and through hormonal regulation they also control the number of eggs being released. It is therefore difficult to estimate how many eggs a 1 kg fish will release on any given night, but over the spawning season she is likely to release between 1-3 million eggs. As a consequence of producing so many eggs, the eggs are very small measuring only 0.7-0.8 mm in diameter (see figure 4).  The egg possesses a single oil globule which provides buoyancy to the egg and essential nutrients to the developing larvae once hatched. Although hatching is temperature dependent the egg will usually hatch within 1.5-2.5 days. At hatching the young bream measures only 2 mm in length, has a large yolk sac, is totally blind and has no mouth or anus. This stage is known as a yolk sac larva (see figure 5). During this time the larva relies on its yolk and oil reserves to develop a functioning digestive system and eyes so it can see, capture and digest prey. On day 4-5 the young bream takes its first meal on a small zooplankton measuring 0.05 mm. Over the next 50 days the young bream develops through several larval stages until it metamorphoses into a juvenile bream.
Juvenile bream grow relatively rapidly until they reach sexual maturity. On Mainland Australia bream mature between 2-5 years of age. Females mature at sizes between 17-20 cm whilst males mature between 14-19 cm (fork length).  However, there are concerns that the size at maturity in some Tasmanian estuaries is much larger than these values. As a result research is underway to assess the size of maturity of Tasmanian bream. After a bream reaches maturity its growth slows down dramatically. For example, Tasmanian black bream grow at an average rate of approximately 100 g a year.  Therefore that 1 kg bumper bream we all like catching is most likely to be around 10 yrs old (see figure 6). So next time you catch a big one think about how long it is going to take before it is replaced. You just never know you may even get a bigger buzz out of seeing it swim away to spawn another day.