Here we feature an article from one of the conference's keynote speakers, Heike K. Lotze, Department of Biology, Dalhousie University, Halifax, Canada, who will speak about the historical ecology of sharks: Reconstructing population changes, ecosystem consequences, and societal values.
Sharks have inhabited the ocean for more than 400 million years, longer than any other marine top predator. Over evolutionary time scales, sharks have also shown comparatively low species origination and extinction rates and thus have been a relatively stable presence in the ocean's past. This presence, however, has been increasingly compromised by a progressive increase in human impacts on shark populations around the world.
To determine the magnitude of change, and the current status of sharks it is important to establish historical reference points and long-term trends. Yet, sharks have left fewer traces than many other large marine vertebrates due to their lower preservation in the fossil and archaeological records, and low commercial value during historical times. The lower quantity and quality of available data makes reconstructing historical changes in sharks more challenging. Nevertheless, the available information from paleontological, archaeological, historical, fisheries, and scientific survey data can help assess long-term changes in shark populations, their ecosystem consequences, and societal values.
Sharks have interacted with people for millennia. Some species have been respected by ancient people in myths and religions, while others have been feared throughout history. Some have been intentionally killed as a perceived or actual threat, or for trophy. Others have been directly targeted by recreational or commercial fishers for a variety of products, from meat to jaws, liver oil, cartilage and fins. In addition, large numbers of sharks have been unintentionally killed in fisheries for other target species, such as swordfish or tuna. Recently, sharks have regained our respect as they are increasingly valued as charismatic and endangered wildlife, important top-predators in marine ecosystems and assets for dive tourism. Many of these interactions have left records that can be used to study the history of sharks.
For example, archaeological records from around the world show that people have used coastal sharks and rays over many millennia, including their teeth, jaws and bones to produce fishing hooks, weapons, and ornaments. In some regions, archaeological remains indicate a decline in the abundance of some sharks over time, while in others people developed technologies to increasingly target larger, faster and more offshore species. Both these trends indicate a change in shark communities over time.
Many historical records, such as those written by early explorers and naturalists, point to the former abundance and large size of sharks they encountered, but also describe the danger and struggle involved in the encounters. Old paintings depict the fear of sharks as well as the wonder, but also describe the methods of catching them with hooks, spears, and nets. Since they have always fascinated people, many old newspaper articles contain records of shark sightings, attacks, and captures that have been used to trace their decline or disappearance over time.
Old fisheries statistics from several parts of the world document the rise and fall of shark fisheries over past centuries and in many cases strong declines in catches or catch-per-unit-effort over time, whether the species was directly targeted or incidentally caught as by-catch. More standardized scientific surveys similarly point to strong population declines as well as range contractions of sharks over time. Today, one quarter to one third of shark species are considered threatened or endangered marine wildlife.
The loss of large numbers of sharks has been shown to entail wide-ranging ecosystem consequences, including changes in other species across multiple trophic levels (e.g. trophic cascades) and decreased resilience and stability of food webs. The important ecological roles fulfilled by sharks are increasingly recognized by people, which has spurred some conservation and management actions. Moreover, a growing number of people want to dive with safe sharks, which has greatly increased awareness, political pressure, and their economic value of being alive. These changing societal values are beginning to have positive effects on sharks in some regions, as they shift human impacts from shark exploitation towards protection and conservation. Thus, there is some renewed hope for the recovery and restoration of sharks, and their continued presence in our oceans.
Heike K. Lotze speaks at 09:00 on Wednsday 22 May at Oceans Past V.
Photo: Sijmon de Waal/Marine Photobank.