Yonathan Zohar

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Yonathan Zohar

Research Interests

  • Basic and applied aspects of fish reproductive physiology and molecular endocrinology
  • Broodstock management and hatchery technologies
  • Aquaculture and fisheries biotechnology
  • Environmentally sustainable recirculating marine aquaculture

The primary focus of my research program is on basic and applied aspects of fish reproductive physiology and endocrinology. A major obstacle for the development and intensification of the finfish aquaculture industry is the failure of farmed fish to reproduce predictably when raised in captivity. We therefore use endocrine, biochemical and molecular approaches to study interactions along the brain-pituitary-gonadal axis leading to reproductive development, gamete maturation, ovulation and spawning. Our research models include commercially important farmed fish, such as striped bass and seabream, and the zebrafish. From our basic research, we develop technologies for the exogenous manipulation of fish reproduction, to be used in the aquaculture industry. These and other major areas of interest are described below.

Endocrine control of reproduction in the brain-pituitary-gonad axis

In fish, the processes of reproductive maturation, gamete production, ovulation and spawning are controlled by hormonal factors common to all vertebrates. In response to environmental cues, the neuropeptide gonadotropin-releasing hormone (GnRH) is produced in the brain. The major function of GnRH is to regulate the synthesis and release of the gonadotropin hormones (GtHs) from the pituitary, which in turn regulate gonadal development. Work in my laboratory has shown that the failure of farmed fish to reproduce reliably in captivity is caused by a dysfunction in the GnRH system. We have also shown that many fish species produce three distinct forms of GnRH in the brain and other tissues, suggesting multiple functions for this hormone. We therefore place special emphasis on the GnRH-GtH axis in studying the endocrine control of fish reproduction. Some basic questions our research has attempted to address include:

  • What is the functional significance of GnRH multiplicity in fish?
  • How does the GnRH-GtH system differ in wild versus captive fish?
  • How are environmental cues transduced into hormonal changes that control reproduction?
  • What roles do the GnRHs and GtHs have in regulating puberty, ovulation and spawning?
  • How do the GnRHs, GtHs, and their receptors function to promote sex reversal?
  • How does input from the environment and the brain regulate the GnRH-reproductive axis?
  • How does feedback from the gonads regulate the GnRH-GtH system in the brain and pituitary?
  • How does local expression of GnRH and GtH in the gonads contribute to gamete development?


The ultimate aim of such studies is to better understand the underlying causes of reproductive dysfunction in farmed fish. From a comparative point of view, our research also seeks to contribute to the understanding of vertebrate reproduction in general.

Early development of the GnRH system

Because of the primary importance of GnRH in reproduction, a critical component to establishing proper adult reproductive function comes during early development of an organism, when the architecture of GnRH-expressing neurons is laid out. Using genetic tools with the zebrafish model, we study this process of GnRH neuron development in fish, in terms of the ontogeny of GnRH expression and the factors controlling proper placement of GnRH neurons and their axonal connections. In addition, recent evidence from our lab and others has shown that GnRH expression occurs very early during development, during organogenesis, suggesting as yet undiscovered roles for this hormone. We are currently investigating the possible roles of the GnRHs during embryogenesis and later in development, when they might be involved in laying down the basic components of the reproductive axis.

Applied technologies for aquaculture and fisheries

A major focus of the work in my laboratory is the application of knowledge gained from our basic studies to the improvement of the aquaculture industry. An early success in this area has been the development of controlled-release, polymeric delivery systems for the administration of GnRHs and GnRH analogs to fish. Use of these delivery systems to manipulate GnRH levels in the blood is able to overcome the hormonal imbalance responsible for the lack of spontaneous ovulation and spawning common in many cultured species. This technology is also used to advance or synchronize natural spawning in order to increase seed production, induce spawning out of season, generate hybrid offspring, or enhance restocking programs, and has been applied in a wide variety of species. Another practical application of manipulating the reproductive axis is the generation of sterile fish. Generation of sterile populations is an important goal in aquaculture for many reasons- sterile fish grow faster, selectively bred or otherwise proprietary broodstock can be more easily protected, and the environmental impact of 'escapees' in cage-culture settings is greatly reduced using sterile fish. Manipulation of the GnRH system offers promise as an efficient means of inducing sterility in fish, and this is a recent line of investigation in my laboratory. Another area of interest is the development of methods to non-invasively administer compounds to fish on a large-scale basis. Injection of vaccines, hormones, antibiotics or marking compounds in an aquaculture setting is labor-intensive, time-consuming, and therefore costly. We have shown that such compounds can be more efficiently administered by combining ultrasound with immersion, greatly reducing the labor involved. These and other practical applications that result from the research in my laboratory are aimed at improving the aquaculture and fisheries industries in order to more efficiently and sustainably provide food fish for the world's growing population.

Recirculating marine aquaculture

The continuous decline of the world's commercially fished species in recent decades has led scientists to conclude that the oceans have attained their maximum sustainable yield, and that global marine fisheries are in danger of collapse. The necessity to farm rather than harvest food fish has become increasingly clear, yet in spite of the significant growth of the aquaculture industry, marine species only account for about one third of total aquaculture production. A major obstacle to the growth of marine aquaculture has been the interaction between current production practices, mainly floating net pens, and the marine and coastal environments. While coastal cages may generate adverse chemical and biological effects on the environment, in many cases the environment in not conducive for optimal growth and health of the species of interest. In response to this situation, researchers at what is now IMET have developed a fully contained, recirculating marine aquaculture system that is able to grow high densities of commercially-important fish using artificial seawater. Most importantly, the unique microbial bio-filtration of this system is able to support high-density, "bio-secure" aquaculture with virtually no water exchange, thus eliminating effluents or escapees and ensuring no interaction with the environment. An additional advantage of this system is the ability to locate marine aquaculture operations in non-traditional settings, such as urban or rural inland environments. This system offers a new generation of aquaculture technology that can be used to economically produce a wide range of marine food fish that are free of environmental contaminants, in a way that is environmentally sustainable.

Blue crab aquaculture, biology and stock restoration

The blue crab is one of the most economically important fisheries species in the Chesapeake Bay region, and a traditional symbol of Maryland. Driven by the recent decline of this species, scientists at what is now IMET initiated an intensive aquaculture program that, for the first time, has closed the lifecycle of this species in captivity. By manipulating the environmental parameters in our aquaculture setting and using intensive larviculture technologies, we are able to mass produce blue crab juveniles year-round for use in basic research in the laboratory and in the field through releases into the Chesapeake Bay ecosystems. This aquaculture program is an important component of a comprehensive and multidiciplinary research program, the Blue Crab Advanced Research Consortium, involving researchers in the University System of Maryland and other universities and institutes nationwide. Ultimately, the goal of these efforts is to facilitate the study of blue crab basic biology and ecology, field experiments, and programs that contribute to the sustainability of this important local species.