Gladiator: Swordfish Life Cycle

Inside look at the unique life history of the swordfish.

Swordfish develop quickly—within one year, this little guy may be 39 inches.

By Juan C. Levesque
Ph.D. Candidate

As a fishery biologist working aboard commercial fishing vessels back in the 1990s, I can still remember the first broadbill swordfish (Xiphias gladius) I saw captured in the western North Atlantic Ocean. Besides its brilliant, radiant blue color, I remember becoming instantly fascinated by the swordfish’s bill; it’s no wonder why Linnaeus named it after the Latin word for sword, gladius, back in 1758.

From the perspective of a modern sportfisherman, “gladiator of the sea” is a great way to describe the swordfish: a powerful, determined fighter, capable of astonishing runs and leaps. And the fact is, science tells us these fish are fighters from day one.

Swordfish larvae primarily feed on fish larvae, but they also consume zooplankton. In the juvenile stage, swordfish have a diet that is very similar to an adult, which consists of squid, pelagic crustaceans, and a variety of fish. Born with a short snout and prickly scales, swordfish develop rapidly. Researchers in the Mediterranean Sea estimated the average growth rate of post-larvae/juvenile swordfish at somewhere between one-eighth and one-quarter of an inch per day, which is similar to postlarval growth rates reported for blue marlin and bluefin tuna. In the Pacific Ocean, the average size of a one-year-old swordfish is around 39 inches eye-to-fork length; most one-year-old swordfish are still immature—big babies, so to speak.

Considered the most widely distributed billfish, swordfish are found in tropical temperate sections of the Atlantic, Pacific and Indian Oceans from about 45°N to 45°S latitude. In the western North Atlantic Ocean, swordfish are found from Newfoundland to Argentina; I’ve personally collected data on these fish from the Grand Banks to Brazil. In terms of year-round concentration, I recorded some of the highest catches of swordfish between Miami and Ft. Pierce, Florida between 1995 and 2001. Given their daily movement pattern, swordfish can be found between the surface and as deep as 3,700 feet. As such, they can tolerate wide-ranging water temperatures (41-81°F). In fact, swordfish have been reported to have the widest temperature range of any billfish.

Movement Patterns

Using pop-up satellite tags, researchers have shown that swordfish display a regular daily vertical migration pattern; they are active near the surface at night and in deeper (980 to 2000 feet) waters during the day. However, swordfish are also known to bask near the surface during the day. Like many species, the daily movements coincided with crepuscular (dawn and dusk) periods. These patterns are also affected by the moon phase.

Swordfish are also known to make long seasonal migrations. Although it is presumed there are resident populations in some locations (e.g. South Florida), many swordfish migrate to cooler waters in summer and warmer waters in winter. Satellite tracking data has shown that swordfish are capable of traveling 1,500 miles in just six weeks (about 37 miles per day). Recapturing a swordfish with a tag is quite an experience. I was on several trips where we recaptured several tagged swordfish. Based on my experience, I found that the most successful fishing captains understood this migratory behavior. They planned their trips and strategies by considering various environmental and behavioral factors, such as the moon phase, water temperature, current direction, swordfish movements, location and time of year.

Diet

Examining countless swordfish stomach contents to assess their diet, I found that swordfish are not generally selective predators. Classified as opportunistic feeders, swordfish consume a variety of species, including squid, pelagic crustaceans, and surface and deep dwelling fish. Swordfish pursue prey throughout the water column. Interestingly, research by Tibbo back in the 1960s proved that swordfish used their long bill to slash at larger prey. Although classified as apex predators, marine mammals and sharks have been known to prey on swordfish.

Age and Growth

The age and growth of swordfish has been challenging to estimate given their small otolith (ear bone), so scientists have used other hard parts, such as vertebrae, or dorsal and anal fins. Removing numerous anal fins with a hacksaw, I know that preparing, reading, and interpreting hard parts is quite an art. Despite these challenges, researchers from all over the world have now estimated the age and growth of swordfish from different oceans. For instance, in the Aegean Sea and Indian Ocean, swordfish have been reported to live up to 10 years, but in the Pacific Ocean longevity has been estimated at 12 years. It’s possible that swordfish can live longer, as these studies were based on specimens collected in commercial fisheries, which can sometimes bias age estimates. More recently, Arocha, a colleague of mine, estimated the age of a female swordfish at 16 years and a male at 12 years; these fish were captured off Venezuela.

Overall, swordfish grow rapidly in early years, but then their growth slows with age, which occurs around age 8 or 9. On average, swordfish grow about 14 inches per year, but there are differences between the sexes. Female swordfish grow faster, live longer, and reach a larger size than male swordfish; almost every swordfish I weighed that was heavier than 200 pounds was a female. It has been reported that swordfish can top 14 feet and 1,400 pounds in weight, but these large fish are rare these days. In my 7 years collecting data on commercial fishing vessels, the largest swordfish I ever saw captured was around 500 pounds; it was caught near West Palm Beach, Florida.

Reproduction

Similar to most fish, male swordfish mature at a smaller and younger age than females. According to a study by Taylor and Murphy in 1992, a small percentage of male swordfish begin to mature at age 1, at which time most of them measure about 39 inches from the tip of the lower jaw to the fork of the tail, otherwise known as lower jaw fork length, or LJFL. Females don’t begin maturing until age 4 (63 inches LJFL). By the time they are 5 (male) and 9 (female) years old, all swordfish are mature. In the Florida Straits, and around the world, swordfish display a protracted spawning season. In other words, active spawning occurs year-round. However, off Florida, spawning activity peaks from April to July. Research by Taylor and Murphy showed that gonad (reproductive gland) development for male (testis) and female (ovary) swordfish peaked in June. They also reported that female swordfish are capable of releasing between 1.4 and 4.2 million eggs; more recent research suggests that swordfish are capable of producing up to 8.7 million eggs. Interestingly, Taylor and Murphy did not find any evidence that larger and heavier female swordfish release more eggs than smaller and lighter females, which is sometimes common in other species. Although swordfish can spawn in multiple locations, they found histological evidence that swordfish spawn in the Gulf Stream between the Florida Keys and Cape Canaveral, Florida. In another study, Lee and Arocha, my old boss and colleague, respectively, discovered that swordfish primarily spawn off Florida, but they also spawn in the northern Caribbean Sea. Although I collected biological data on many ripe female swordfish in the Florida Straits, large females always seemed to gather in specific areas of the Caribbean, such the Windward Passage between Cuba and Haiti during fall through winter. Based on tagging, histological, and larvae abundance data, it’s evident that swordfish spawn in specific areas, such as the Florida Straits.

Fishery Management

In the United States, swordfish are targeted by commercial and recreational fisheries. Given their economic and social importance, management of swordfish and other Highly Migratory Species in the
western North Atlantic Ocean is a multilayered process that involves both domestic and international governing bodies; working as a fishery manager for NMFS in a previous life, I can attest that it’s a complicated process. The primary international governing body is the International Commission for the Conservation of Atlantic Tunas (ICCAT), which is headquartered in Madrid, Spain. In the United States, swordfish are managed by the NMFS under the authority of several Congressional Acts, such as the Magnuson-Stevens Act and the Atlantic Tunas Convention Act. The NMFS implements fishery management regulatory measures under the guidance of ICCAT and other advisory groups, such as the HMS Advisory Panel. The latter is a diverse group of stakeholders and experts who are knowledgeable about Atlantic HMS and/or Atlantic HMS fisheries; HMS AP members serve three year terms. Currently, swordfish are managed using catch quotas, size limits, time/ area closures, and gear restrictions.

The commercial fishery began with a few fishermen targeting swordfish during the day with harpoons in the 1920s, but the fishery never fully expanded until the development of specialized commercial fishing gear (i.e., pelagic longline), which occurred in the 1960s. Directed commercial fishing effort continued to increase during the 1970s through 1980s, and by 1987 swordfish landings peaked in the western North Atlantic Ocean. Shortly thereafter, an evaluation of the swordfish stock revealed it had significantly declined and was at the verge of possible collapse. To recover the stock, ICCAT immediately implemented a 10-year rebuilding plan for swordfish in the Atlantic Ocean in 1999, which included reductions in the total allowable catch. In addition, the NMFS instituted a series of time area closures in the Gulf of Mexico and the Southeast region of the United States to prevent the bycatch and discards of HMS species that included juvenile swordfish.

Today, due to the stringent fishery management actions, reduction in commercial fishing effort, and strong recruitment, the most recent stock assessment for swordfish in the Atlantic suggests the stock has recovered and it’s at, or above, maximum sustainable yield. Given this recovery, swordfish once again support a productive recreational fishery.

As the first biologist to document the recovery of the recreational swordfish fishery off South Florida back in the early 2000s, I described the basic gear and techniques anglers used to target this highly migratory species. Today, these tactics have dramatically changed with the development of the daytime deep-drop strategy, which has produced some very large swordfish and helped expand the fishing guide service in South Florida. FS

First Published Florida Sportsman Sept. 2014