Was mortality higher on French slave ships and if so, why?

SHORT ANSWER

Basically, NO.

The two main databases on the trans-Atlantic slave trade do not support the idea that French slave ships had, on average, higher mortality rates than American carriers. In fact, data suggests that the opposite is probably true. This was most likely due to American slavers being less experienced at handling slave ships than the British and the French.

The main trend on mortality shows a decline over time for most flag carriers. The generally higher mortality rates for the early period of the trans-Atlantic slave trade can be attributed to slow ships (=more time at sea), worse conditions, and inexperienced crews.

DETAILED ANSWER

Shocking though the Atlantic slave trade was, the mortality statistics cited in the question may not tally with other sources. The data quoted here comes mostly from the Du Bois Institute dataset (in ‘Transoceanic Mortality: The Slave Trade in Comparative Perspective’ by Herbert S. Klein, Stanley L. Engerman, Robin Haines, and Ralph Shlomowitz) and the Voyages Database (from ‘The Trans-Atlantic Slave Trade: A Database’ published by Cambridge University Press). These two sources seem to be the two most comprehensive sets yet compiled.

The total number of slaves shipped to the Americas is variously estimated to be 12 million, 12.5 million, and 13.9 million (Edward Dunber, cited in P. Curtin).

The three largest flag carriers between 1501 and 1870 were the Portuguese (around 5.8 million embarked), the British (around 3.2 million embarked), and the French (around 1.4 million embarked).

Estimates of mortality rates during shipment vary from 12.5% to around 16%. Mortality rates decreased over time from around 21% during the period 1601-1650 to about 6% for the period 1751-1800 (during which time the trade peaked), and then increased during the period 1801 – 1850 to about 13%. The decline was partly due to improved conditions (though probably for economic rather than humanitarian reasons) and faster ships, meaning less time at sea. The increase at the tail end of the slave trade was mostly due to the Portuguese who accounted for over 75% of the trade in this period as Britain (in 1808) and France (in 1815) had outlawed it.

These figures do not take account of deaths during the acquisition of slaves as no one has been able to accurately estimate this.

As far as mortality rates by flag carrier are concerned, neither set of statistics I’ve found support the idea that rates were significantly higher on French ships. For the period 1501 to 1870, calculations from one source gives rates of

• 16.2% for Britain
• 15.7% for France
• 17.3% for the USA

The other source (Transoceanic Mortality) gives

• 12.5% for Britain
• 13.3% for France
• 16.9% for carriers which were mostly American and Scandinavian ships (separate figures are not given)

It is true that the British passed legislation which limited the number of slaves a ship could carry but this was not until 1788 (The Dolben Act), by which time British ships had already transported more than 2.5 million of the estimated final total of 3.26 million slaves.

These rates seem very much at variance with the proportions quoted in the question. Either the museum presented ships with higher than average mortality rates, and / or they included deaths for a certain period either before and / or after embarkation and disembarkation. I think the most likely reason for the apparent discrepancy is that the museum focused on earlier ships when mortality rates were much higher. The vast majority of slaves were shipped during the period 1700 to 1850, by which time death rates had been as much as halved compared to the pre-1700 period (for both slaves and crew).

If we look at mortality rates for earlier periods of French slave trading, we can find numbers which approach a 25% mortality rate. According to :

French slavers left Africa with similar numbers of slaves onboard before 1716 as they did after that year (326 on average, compared with 328 after 1715), but the earlier group experienced almost double the number of shipboard deaths than did their later counterparts.Almost one-quarter of those embarked before 1716 died before reaching the Americas, compared with one in eight of those carried off between 1716 and 1792.

Source: James Pritchard, David Eltis, and David Richardson, 'The Significance of the French Slave Trade to the Evolution of the French Atlantic World before 1716' (2008)

The authors further state that

This discrepancy partly reflects declining shipboard mortality over time, but the declining overall trend in mortality was not so large that it can explain all of the differential,and it seems that the latter may also have been due to the lack of experience of French crews at this time compared with Portuguese or English slave traders. Such a rate is similar to that found on Portuguese and Dutch vessels in the second quarter of the seventeenth century.

Higher death rates were also evident on early British slave ships e.g. 1601-1650 mortality rate was 22%, 1651 to 1700 it was almost 24% (see Voyages Database). Declining mortality rates were not just applicable to slaves but also to the crews that manned these (and other) ships.

What cannot be disputed is that the treatment of slaves after arrival was far worse for those working on sugar plantations in the French and British colonies than those who were in the US. Mortality rates were indeed significantly higher for the former.

Other sources:

Philip D. Curtin, 'The Atlantic Slave Trade: a Census’

Jones, A. and Johnson, M., 'Slaves from the Windward Coast', Journal of African History. 21,1 (1980), pp.17-34.

Roberts, R., 'Warriors. Merchants and Slaves' (Stanford University Press, Stanford, 1987).

Will Hardy, 'The Rise and Fall of the Slave Trade'

Patrick Manning, cited in: https://en.wikipedia.org/wiki/Atlantic_slave_trade#Atlantic_shipment

Slavery: The Middle Passage

Description of a slave ship, about 1788 (Understanding Slavery Initiative)

David Eltis and David Richardson, 'Extending The Frontiers: Essays on the New Transatlantic Slave Trade Database' (2008)

French slave trading practices were more abusive than comparable American practices for several reasons.

1. The French sent out more and larger ships than the Anglo Americans. Conditions were much worse with overcrowding, etc. than on smaller ships.

2. The destination for French slaves were Caribbean sugar islands, which were more nearly comparable to similar Spanish colonies, rather than more temperate climates such as Virginia. Because French slaves died off at a much greater rate than those of American destinations in any event, the French "shippers" were less concerned about delivering slaves in "good" condition. From the link:

Conditions on sugar plantations were harsh (though French sugar colonies were no better or worse than Spanish, Dutch, or British ones). During the eight-month sugar harvest, slaves sometimes worked continuously almost around the clock. Accidents caused by long hours and primitive machinery were horrible. In the big plantations, the captives lived in barracks; women were few and families nonexistent.

Compared to this, North American cotton plantation slavery featured much less ferocious labor and allowed family units to exist. Which is one reason France required a steady flow of thousands of slaves a year -- to replace the ones the French had worked to death -- while America's slave population grew naturally even after the U.S. slave trade had ended.

3. The movement for "Abolition" started earlier in France, 18th century vs. 19th century, which is to say that opinion became more polarized, sooner, in France than in America.