Ships carrying animals appear to be at more risk because of a number of factors. Estimates suggest that least 80% of livestock carriers were originally constructed for other purposes and have been converted, rather than purpose built. “Historically, the majority of livestock carriers have been converted container ships and ro-ros [roll-on/roll-off vessels]. But even passenger vessels and oil tankers have been turned into livestock carriers,” said Capt Samson Rathaur, senior marine risk consultant with Allianz Global Corporate and Specialty.

Adam Kent, managing director of market analyst Maritime Strategies International, offered similar figures. “Only about 20% of the current livestock fleet was originally designed to carry livestock,” he said.

The age of the world’s livestock carriers can also be a problem. Rathaur said many were often already near or at the end of their shipping life, making the fleet “one of the oldest sectors in shipping”. Kent estimated the livestock fleet’s average age at 36 years, 16 years more than the 20-year average age for the global merchant shipping fleet.

All of the livestock ships lost in this period were originally built for another purpose. In one case the ship – a ro-ro cargo vessel – had not even been converted to carry livestock. All but one of the livestock carriers lost were more than 20 years old.

Owners of higher-end livestock carriers argue against tarring all ships with the same brush. For example, Livestock Express, which owns 13 purpose-built ships and one conversion, says the average age of its ships is 14 years.

Livestock Express is owned by the Dutch company Vroon and is described as the “world’s biggest independent seaborne livestock carrier”.

Asked about higher livestock ship losses, Paul Pistorius, the company’s managing director, said he was “obviously aware” of a number of tragic incidents over the past 10 years. “All of these casualties have involved converted vessels, many of which have been older than 30 years.”

Kent said that while the Guardian’s calculation was “sound” and seemed to indicate “something amiss in the global livestock fleet”, the risk applied mainly to older conversions.

He said that “some very good quality conversions” existed, “though we don’t tend to hear about these ships very often, only about those with which there are problems”.

Another significant factor is that live animals move around in transit, unlike containers. “When these vessels are converted, animal behaviour is not always a factor in their design,” said Rathaur. Switching to a moving cargo without due consideration could affect a vessel’s stability, he said. “During heavy weather, animals can get distressed and move together to one side, which can potentially list the vessel.”

The tiers that many livestock conversions add to increase carrying capacity were another stability risk, he said. Manure and urine also had corrosive effects on steel. Fermenting manure releases moisture, ammonia and carbon dioxide. In conjunction with chloride naturally excreted by animals, the resulting slurry could be “very corrosive to steel structures and ill-maintained ships and can, over time, have a serious impact on the structural strength of the decks,” Rathaur added.

He said livestock shipping also had a human resources problem, because crews working on livestock carriers might have “little, if any” training or experience in livestock management before joining their first livestock ship.

Using data from the Paris MoU shipping inspector, the Guardian found the most recent three-year rate for livestock ships being detained after inspections was almost 9%. For general cargo/multipurpose ships that average was just less than 6.5%.

The Paris MoU is a memorandum of understanding between 27 countries that aims to eliminate substandard ships. It carried out almost 18,000 inspections in 2019. Detained ships are usually kept in port until problems identified by inspectors are resolved.

The International Chamber of Shipping (ICS), a trade association, would not comment on the higher risk of loss for livestock ships exposed by the Guardian. In an emailed statement, the ICS marine director, Jonathan Spremulli, said: “We need all incidents to be thoroughly and robustly investigated and detailed reports to be made available as soon as possible so that lessons can be learned and appropriate actions taken to prevent similar occurrences.”

Spremulli added: “The ICS continues working with the International Maritime Organization (IMO) to ensure ships are built, maintained and operated in accordance with a robust regulatory framework which is continually under review and being improved.”

In many, but not all cases, ship loss investigation reports are filed in the IMO’s Global Integrated Shipping Information System marine casualties and incidents database. Of the seven livestock ships lost since 2009, three reports are currently available. Two mention animal movement as an aggravating factor in already difficult circumstances.

All three reports contain safety improvement recommendations. Most specifically, the report for the Danny F II, lost in December 2009 with the deaths of 40 crew and 28,000 animals, suggests a set of rules be established “for the construction and/or classification of the Lifestock [sic] carriers” because a “large number of livestock vessels currently in service are the result of a conversion from different type of original designs”.

A spokesperson for the IMO said the suggestion had been considered by its maritime safety committee, but, for the “IMO to address this, a member state or states would need to submit a proposal for IMO to look further into this issue”.

Sign up for the Animals farmed monthly update to get a roundup of the best farming and food stories across the world and keep up with our investigations. You can send us your stories and thoughts at animalsfarmed@theguardian.com